I guess none of this makes sense to me because when I am with someone, I don't have a desire to be with other people. I've been in relationships with very average looking guys, and when a hot guy would hit on me, I'd feel a small twinge of desire and perhaps a fleeting sense of regret that I was attached, but it never went beyond that. It doesn't do anything for me to fantasize about a guy I have no connection to, and so, whenever I am attached, I fantasize about various situations with my partner, and usually no one else.
I think it doesn't makes sense to me why, if you're truly happy with what you have, and that person is absolute for you, you'd be focusing time and sexual energy on someone else - unless you were unsatisfied with or unfulfilled by your partner. I think it will probably always bother me that, quite unlike women, men never seem to be content with just one person. I'm not saying it bothers me that they find other women attractive or desirable - it bothers me that they feel compelled to 'have' those women in some way.
I guess in a really perverse way that is comforting- but again, it just reinforces to me that I must in some way be inadequate in his mind if it is so important to him that he be with other women besides me. Maybe because I feel that if a man was really invested in a woman, he wouldn't be concentrating so hard on other women. I feel they would be a momentary distraction, and not the object of a sustained fantasy.