Why not ask what a man is thinking about? Why is it a turn off? Just trying to get inside the male psyche, here.
I thought it was a universal understanding that most men, myself included, just don't like talking about our...*feeeeeelings*.
The honest answer is, because many of use are too insecure to say something that we know will make you mad, even if it's true. Hell, even if you've just had sex, you could be thinking, "You know, I took my car to the guy just last week, and the friggin' thing is still acting up. Four HUNDRED dollars it cost me and the damn thing is shifting like a #$%^ GARBAGE TRUCK and he says it's just the weather..." And she asks, all dreamily and sappy, "Whatcha thinkin' 'bout?" And we say, "Uh, nothing."
The funny answer is, because no matter how old, or what the situation, all men think the same things, all the time; "I'd like a beer and I'd like to see somethin' nekkid."
Look at it this way; how would you feel if, in the middle of a warm and loving hug, he asks you: "Did you pick up that alternater from the car-parts store?"
To a male, generally speaking, sex is more a physical thing than an emotional one. He would rather have your warm embraces and passionate responses than your attempting to compare the moment with a romantic poem or two.
That, and such moments are when the female typically attempts to maneuver the conversation about in order to determine the 'extent' of his emotional attatchment. All men become nervous at this stage, as an incorrect answer may very well result in the immediate cessation of any further embraces -to say the least- for that time or conceivably any other.
How old are you, dear? Seriously, they are tired of saying "Nothing" and you not believing them. Anyway, you don't want to go there. Looking into a man's mind is like watching sausage being made.