Sex is not always on the male mind, but it doesn't take much to bring it to the surface. A halfway suggestive comment, a glance at an attractive girl (or woman), a particular scent in the air, the fact that it's Tuesday... all sorts of things can make a guy think about sex. I think that guys are far more easily stimulated to think about sex when compared to girls, and since they will therefore be ready to say something sexually oriented no matter what the occasion, it seems as if it's on their minds all the time.
And it doesn't help that one is almost constantly bombarded by sexual images in advertising and popular culture. Watch any channel on TV at prime time for an hour, and you'll see at least one girl in a bikini. I once drove across the city where I live and counted over twenty advertisements for strip clubs visible from the road. We don't stand a chance!
Guys also tend to judge each other based on their sexual prowess - we don't hold James Bond in high esteem because he's a snappy dresser, we do it because he scored with Pussy Galore! Boys who have a lot of sex are considered Studs, and win the admiration of their peers; girls who do the same are considered Sluts, and are looked down upon. It's a stupid and asinine double-standard that makes no sense, but that's the way things are. HOWEVER, as boys grow into men, they begin to understand that there are better ways to measure masculinity. Obsession with sex and the pursuit of "studliness" is really a sign of immaturity.
Personally, I'd say that at this point in my life, I probably spend about 10% of my waking hours thinking about sex; and I admit I'd probably do it more often if I didn't spend a lot of time at work where I have to concentrate on fixing computers. But don't judge the boys too harshly - I remember very well those raging hormones, and they do NOT make it easy to maintain discipline.
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