I can add a little now, and more when I get back from bringing yoghurt to a neighbot whose only recourse against a lung cancer death now (because he refused medical attention & it's too late for chemotherapy or radiation) is an alcaline diet and some other alternative treatments. It's not unusual for the sexual intensity among people in their 60s to decline and even cease altogether. The two of you seem to have picked opposing strategies for dealing with it: withdrawal, which Michael calls "sitting back" and boiling frustration leading to attacking the other verbally, which Debbie neglects to mention. but the two responses also mutually intensify each other.
It's also quite common for the man's retirement to trigger a marital crisis like what you're experiencing, because now the normal limitations on time spent in each other's presence are gone, and the structure of your relationship leaves no room for absense to make the heart grow fonder. So you have a vicious circle where each person's unsatisfying response triggers the other's, and each sees the log in the other's eye but not in their own. I'm in the same age range as you are, and so is my wife of 27 years, so I know whereof I speak.
The roots of your problems may lie in some physical conditions related to sex, but what's maintaining them is a cycle of emotional mismatching, and the fundamental emotions whose handling is derailing your intimacy belong to the family derived from the inborn emotion whose technical name is SHAME. This family includes shyness, awkwardness, embarrassment, disappointment, vulnerability, hurt, guilt, humiliation, and "feeling bad" to name the most common. The shame family of emotions can be triggered by any situation that has made us shy, awkward, embarrassed, etc before. But it arises anew in any situation where one is feeling interest/excitement or joy/happiness, whenever something interrupts and impedes the excitement or joy but doesn't completely eliminate either of them.
Shame emotions are Extremely Frequent in Love relationships, as shyness, disappointment, vulnerability, hurt, guilt and humiliation, because Love itself shows up most often as interest/excitement (where interest is what makes us pay attention and excitement is its most intense expression) and joy or happiness. So any time the interest & joy we feel in our partner's presence (or even when we think or imagine about him or her) gets interrupted, a shame emotion is triggered: The evolutionary purpose of this shame emotion is to put the brakes on the positive emotions, so we can pull back a bit and revise our thoughts and actions in order to get back on the right track to harmoniously relating again. Fear warns us against physical dangers, so we'll stop, look and listen before proceeding; shame warns us against mismatches in our social interaction, so we can pause, step back and make adjustments in interpersonal situations.
The problems in our intimate lives are not due to (what I call) "shame moments," because shame's healthy purpose is to stop our excitement- and joy-driven interactions when we're not on the same wavelength, so we can make helpful adjustments before moving ahead with each other. A Shame moment is like a warning light on your car's dashboard: Check engine, or Refuel. Our relationship problems arise because of four ways we normally react to shame moments that don't necessarily fit with what our partner is doing:
1st, We WITHDRAW, which is instinctual, because in the first moment of shame/shyness/etc we can't think straight; so we need to pull back from our interpersonal situation to collect our wits and THEN figure out what to do to rectify the situation. :
2nd We may "Attack-Self" that is blame ourselves for the discomfort in our situation. This can be helpful because it encourages us to look for what we might be doing to make the situation uncomfortable; and it may also prompt us to openly take the blame and apologize or try to make things better, or lower ourselves to get the other person(s) to forgive us. This can also be depressing, and it can lead to internally "beating ourselves up" and at worst, suicide. Girls and women are more inclined to do this, which leads them to apologize and to try harder than boys and men to find ways to improve inharmonious and uncomfortable situations.
3rd, We may Avoid the feelings of shame, either by refocusing our attention onto something else that provides excitement or enjoyment, or by engaging in play, alcohol or drugs that make us feel good instead of uncomfortable--which is why 95% of my college students wrote in papers about why they first used alcohol (or pot) that it was because it made them unable to feel shy, so they could talk to the opposite sex. Alcohol is called "liquid courage" because it interferes with the brain's ability to feel both fear and shame. We can also AVOID feeling uncomfortable around other people if we carry around a superiority complex, believing we're better looking than others, richer, better educated, nicer, more important, or special in any other way.
4th, We may Attack-Other and avoid feeling shame-discomfort by blaming someone else for the social glitch, or just flipping into anger at anyone or anything, because anger makes us feel strong, while shame makes us feel weak.
One more point before proceeding to advice for how to cope more effectively with your shame moments, which seem now to be overwhelming the intimacy and love you both want to continue having--which is why neither of you has filed for divorce.
If Loving is a prime emotional state for triggering Shame moments with every glitch, SEX is even more likely to spawn shame moments like sticking a sparkler into an anthill. For sexual arousal intensifies excitement and enjoyment even more than love. So sexual arousal has been ringed with shame triggers ever since early childhood--and the natural WITHDRAWAL response to shame applies to shutting down and withdrawing from sexual arousal with the slightest glitch in mutual erotic stimulation. That's why sexual activity must be so private for most people, and why the conditions for each partner being able to enjoy their arousal without repeatedly shutting down can be so tricky, and both partners need to exercise extreme care and compassion to keep the flames of erotic activity from suddenly snuffing out.
So now we're finally ready to approach your present self-sabotaging emotional cycles with regard to both sex and other intimate issues. Michael, you report feeling criticized by Debbie: if you feel it, then it's how you feel, whether Debbie wants to call "having a different opinion" "criticizing" or not. You can't change how your partner feels by telling him that he's wrong about what he's reacting to. If you (M) feel hurt (that is, your expression of your ideas and beliefs is negated) when Debbie counters your statement with a different one of her own, then you've become uncomfortable, and your discomfort will probably rise if her disagreement is vehement--as it's more likely to be emotionally intense if she's living with a lot of underlying frustration of her hunger for your touch. Debbie, you are in Attack-Other mode, which may be only a tone of voice that for you is justified because Michael doesn't seem to be trying hard like you are to make things better between you. In fact Michale is WITHDRAWING because he's probably learned way back in his childhood that pulling back from any dispute with other people works pretty well to cool any hot situation down; and chances are good that you (M) learned your withdrawal as a way to keep from running into your mother's dominance and temper (or perhaps your father's).
I'm worn out now, so I'm not going to work on how to change your patterns until tomorrow. But I will suggest this in the mean time. Each of you, separately, start recording WHEN you personally have a shame moment that occurs in the context of your partner-interaction, either person-2-person or in thinking about your partner. Take pauses in your day and write down very briefly what the "shame moment" was (1 sentence if you can keep it brief), the names you'd give to your shame emotion, and then which of the 4 coping responses you engaged in, including often 2, 3 or even all 4 of those possible (sometimes "attack-self" or "attack-other" don't get expressed outwardly, but they happen inside of your own head; but that doesn't make them any less real or important in your experience. Then finally, spend a little thought&writing time guessing at what else you might do in the situation you've just recorded if you wanted to adjust YOUR actions & thought so your interaction MIGHT go more smoothely next time.
Notice that one way to detect a lot more Shame Moments than you would expect at first to be having is to note each time you 1. WITHDRAW from engagement with your partner, 2. BLAME YOURSELF or FEEL DEPRESSED, 3. DISTRACT YOURSELF by quickly shifting your attention to something absorbing, like your Computer, TV, or when you reach for food or drink, or 4. BLAME or HAVE NEGATIVE THOUGHTS about your PARTNER, or ESCALATE AN ARGUMENT. For you can backtrack from every occasion of 1, 2, 3, or 4 to the Shame Moment that came right before it.
Notice also that Shame Moments are not to be blamed on the other person, or even discussed with her or him until you've learned what you can from your own shame moments FOR YOURSELF. You're not going to stop doing 3 or 4 of the coping types, but you can go back afterwards to the Shame Moment itself and then brainstorm about how you might respond differently to improve the situation for both you and your partner. And THOSE reflections will eventually bear fruit in putting out the brushfires that are currently making your intimacy like sitting on a hill of fireants.
This is the core of a couples relationship mending program I developed and field tested with my college students and also with couples in short-term counseling almost 10 years ago. I have more to write about it, esp because your extreme discomfort around sex needs more than just the mending influence of your shame moment awareness. The next steps up from what I've described would be Sex Therapy if and when you're ready to trust each other enough to open up to sexual arousal. (I assume that you, Michael, feel very uncomfortable and vulnerable about something involved in your sexuality, and you don't want to discuss it at all with anybody, even though that's what urologists do every day. You've probably dealt with very uncomfortable life situations through withdrawal before, so that's how you're dealing with it now.