It's during the actual act that we can finally let our minds roam. We begin to free ourselves from the physical, logistical mechanical aspects of sex and allow the psychological sides to emerge. Once you get into that rhythmic thrusting, you can let your mind go a little. But it's hard for men to nail down just what they're thinking about and when they're thinking it. The images that go through your head are like pepper in soup.
Fantasies mingle with reality and it all becomes one.
Contrary to what you might think, men even men in long-term relationships generally don't fantasize about making love to a different woman. Unless a guy is really in love with someone else, rest assured, you're the star of his X-rated show. But they're fleeting. For instance, if the woman I'm with has short hair, I might imagine that her hair is long and falling down against me, and I'm running my fingers through it. Sometimes if I do it, I'll even begin to lose my erection.