Well, this is a fine kettle of fish. I went thru something like this in 1971-72, when my first great love was in its 8th year. We were both 29. There is so much more that I know about these erotic/emotional forces that I know now, since my experience with therapy began then and I went to psychology grad school in 1973-75 and changed my profession to psychotherapy (from Humanities, English & German literature, where I had my first PhD.
When we did this it was the cool new thing to do, with the books The Harrad Experiment (Heinlein) and Open Marriage coming out right then. Now I've heard there's another wave of this sort of experimentation among college kids, they call it Polyamorous, where it's not just one or two threesomes, but each person might have 3-5 lovers, mostly pigeonholed to Internet & phone calls, other ports of call, and "old friends," etc.
I think I know where your own sexual arousal patterns originated. But for now, this is not the most immediate challenge. What other close relationships and/or sexual relationships does Tom have?
You've cut a deal without halting the experiment in New Love, by asking for and getting some reassurance that you have the power to limit the duration of their amorous behavior, and that's probably slowed down your potential of losing your mind for the time being. But now you have a relationship problem, and it will only get bigger and more entrenched unless one of two things happens. Either Margie falls out of love with Tom, or you have a change of heart in your conscious attitude.
You wrote: "For her part she is willing to stop seeing Tom, but she does not want to since it would hurt Tom very much and she cares about him a great deal. They both have a strong emotional involvement. Having said that Margie has reaffirmed that I am the most important partner to her."
The fact that you feel guilty about depriving Margie of her new love would be touching if you were her father, brother, or son, but it does not bode well for the future of your marriage as her husband. It is really great that you can express all of your feelings with Margie, because it means your intimacy with her is very strong. And actually, if you continue to be able to express what you're feeling with her, and you are ABLE to keep expressing all your feelings, especially feeling left out and hurt, she might fall out of love sooner rather than later. But there's one area of feelings you're not sharing with Margie, because you're probably not aware of them yourself. And that's Anger.
If you thought I was fishing for sexual violence when I asked you if you ever felt aggressive during sex, it was quite the opposite. If a man can't spontaneously feel like an aggressive animal during sex with his mate (sometimes)--and a lot of men can't--then he's missing something in his emotional make up. I was missing that myself during my 20s, and it led me to let my girlfriend's new romance with a 19 yr old student (my favorite young guy, like my son) go on, even though I could feel often enough how much it hurt me to lose her to her excitement for him. I was unable to get angry or aggressive enough to assert my partnership with my girlfriend and demand she stop fooling around with the kid, or declare that I would make him leave our socalled communal house. It took about 9 more months, but I could never get back the sense of belonging and security I once had, and the relationship was doomed.
Anger is a manifestation of the natural male aggression that leads men to hunt their women down, to protect them from all dangers and also all rivals, and to feel triumphant, possessive and masterful when they are thrusting uninhibitedly in the climactic stage of male orgasm. [I'm particularly concerned about your reactions to this little paragraph, because it's one of the vital centers of this issue.
You're not knowing mentally and not reacting instinctively with "mate guarding" like the other male primates. So "New love chases away old," (The Art of Courtly Love, Andreas Capellanus, 1174, southern France) no matter what Margie tells you about you being number one husband. The rich Arabs can add another wife every 5-7 years if they can afford to support her. So they'll go on according number one wife status to their first wife, but their passion is bound to flow towards the new woman in their life. If you allow Margie and Tom to keep on heating up their chemistry, you're guaranteeing that their new love will take months to a few years to simmer down, and the limitations you put around it for your own version of "mate-guarding" will either make their passion work harder to work itself through their minds and hearts (like the famous adulterous affairs of Tristan & Isolde, the forbidden passion of Romeo & Juliet), or perhaps falter then fizzle if there's enough guilt in either Tom or Margie.
You wrote: For her part she is willing to stop seeing Tom, but she does not want to since it would hurt Tom very much and she cares about him a great deal.
How much MORE is it going to hurt Tom if she waits 6 months or more likely 2-3years to stop seeing him? How much LESS of her number one man are you going to feel as yourself if you watch it continue for ONE more month? I'm very concerned that you and I don't have enough trust built up for me to guide you through what's coming your way. This written Q & A approach between strangers is not strong enough, not multichannel enough to go into the deeper stuff that you've sculpted into the sex fantasies and sexual behavior you're now acting out. For most people sex fantasies are kept on the imaginary level, because once they're enacted in real life they have a tendency to become more compelling than any other emotional sector in the love relationship. Sexual intensity threatens to sweep away the more tender love feelings, and people can lose their grip on the identity of themselves, their partner, and their love relationship itself.
Our species is biologically designed to carry on serial monogamy, and in the vast majority of cultures it's the men who can get away with having a lover or two on the side, especially if their first wives don't know or choose not to know about them. Women have to be able to warm up to another man, because in the bad old days their husbands could be killed fighting another tribe. I