That is an excellent question, thanks for writing. In general, sexual intimacy is extremely important to a relationship because it is (literally) the only "need" that gets 100% satisfied between the two of you. Other folks can feed you, clothe you, give you a hug and a kiss... but it is only each other that satisfies your sexual cravings. Whether it is together through intercourse or alone through masturbation, sex is completely held within the internal walls of the union.
Now having said that, let me also say this... there are plenty of couples who replace actual SEX with other means of physical reassurance and intimate expression. Holding hands, rubbing each others' backs, lying in the spoon position... these can act as the glue that holds the marriage together when sexual intercourse is no longer possible. And make no mistake about it - as men age, their ability to achieve and/or maintain an erection begins to fade with time. In fact, we call it "The Rule of 10s." About 60% of men in their 60s cannot support a constant erection, about 70% of men in their 70s, 80% of men in their 80s, etc.
The problem here is that often times, a man's feeling of self worth, self-esteem, and masculinity is tied to their ability to perform sexually. Don't ask me why, it's just one of those things. So, when the erection begins to fail them, they go through a crisis of identity ("Who am I to my wife if I am no longer manly enough to have intercourse with her?"). Your reassurance may not be enough in the short run as he is going through his adjustment period, but stick with it and help him to feel loved, needed, and USEFUL to you without fulfilling you in that way sexually.
Then, also let him know how he CAN satisfy you physically. Not necessarily though intercourse, but there are a lot of things he can do with you and for you if your sexual needs are out-pacing his right now. And let's not forget about the medical options for erectile dysfunction... Viagra, Cialis, and Levitra have infused hope and new life in relationships of couples who are entering their golden years.
Best of luck to you, and regards XXXXX XXXXX husband. If you are satisfied with the response, please hit "Accept." That is the only way I receive credit for my answer. Thanks-