Don't tell my wife, but lately I can only cross the finish line by visualizing premarital sexploits with hot girlfriends of decades ago. (I guess those gals, now in their seventies, would be surprised at how they're helping my marriage now!)
In addition to what we pick up in our younger sex lives, in our conscious minds, if you go through the thousands of past comments in FT, you'll find that we guys have all kinds of hidden stuff in our subconsciousness. Seems to be hard-wired for life, like instincts in the animal world. There's a fun book "A Billion Wicked Thoughts," in which the authors analyze a billion Internet searches to discover what arouses guys (and women).
(For an exclusive peek into the sexual subconsciousness of women, there are Nancy Friday's books "My Secret Garden" and "Forbidden Flowers.")
We have "crazy" stuff, hidden and unhidden, in our minds, which might cause us to think/say/do "crazy" things when having sex, and it's normal!
So why don't I confess my nifty fantasies to my wife? We're very open about sex usually. Because she's insecure after being cheated on by her ex, and she knows I have wandering eyes!