Here's one response to my question about what role fantasy plays in your life .
I would say that I use sexual fantasies in two separate ways:
1) I use them when I am having sex. I am married and have always been monogamous, and my husband had an affair early in our marriage. I find myself fantasizing about his having sex with other women when we are having sex. I would say I do this regularly, probably every time, but the fantasies tend to be brief: mere flashes really. I do not have a good visual memory so I fantasize (and think in general) in "impressions." I also have violent sexual fantasies while having sex, and they tend to be more prevalent when I'm actually aroused (which I would say I am about half the time), and the closer I get to orgasm, the more violent they become, especially if the orgasm is difficult to attain. By "violent," I mean I have rape fantasies where I am being raped (almost always) by a man, and unless I put my husband in that role (rare), the man is always someone I do not know, although in the fantasy he would not necessarily be a stranger. Again, these fantasies are brief, although it's difficult to say how long since the whole thing is very situational. I would say no longer than maybe 20 seconds, although I'll have several in a row or replay the same one over if necessary.
2) I use them when I masturbate. I would say these are pretty similar, although since I am more likely to have an orgasm when I masturbate, and since it's quicker and less intimate, my fantasies tend to "run" quicker in my head (same fantasy, faster show time). I have a few fantasies that are in regular rotation for this purpose, and often I do not think about my husband at all, even to imagine him with another woman, which is (as I said) something I do when we are together.
General comments: I always fantasize at least some when I'm having sex or masturbating. I never fantasize out in the "real world," and I never fantasize about famous people or even people I know (other than regarding my husband as I've otherwise mentioned). Most of my fantasies are modified from a scene I've seen on TV or in a movie, for example, I use the scene from Excalibur where Arthur's father transmutes into the form of another man and has sex with Arthur's mother while wearing bloody armor; and (I'm ashamed to say) the rape scene from The Accused .
In my fantasies, I equate male violence with actual desire. I do not personally like "romance" when it comes to sex. (Rose petals on the bed? Come ON. I'd just think about how I would have to clean them up.) I prefer passion, which I equate with aggression. Not to get all "therapy session" on you, I will say this: My husband's affair was very early in our marriage, and I have always secretly assumed it was because I am inadequate in one or more ways sexually. As a result, when he wants sex, I always assume he wants sex, not sex with me particularly, so I substitute what I feel like I'm missing from him (a genuine desire as evidenced by passion and a lack of control) with fantasy.