I really, REALLY don't feel like typing up another essay, so I'll just leave it at this: We think like that because of testosterone.
Never mind, now I feel like elaborating. It's completely natural to associate things with sexual references. Our hormones give us that ability. However, the gravity of these thoughts could very well be due to the odd role sex plays here in America, as opposed to other countries. In Europe, it's an everyday thing to see some topless model in a fashion magazine. Here, however, the government bans it. In Europe, you could walk up to any random stranger, have sex with him/her, and walk away with no qualms. Here, the government seems to think that doing this is against the so-called "moral code." Needless to say, our country has a lot of work to do on accepting the role sexuality plays in everyday life, and not trying to abolish any sense of it.