Here's the easy answer: Because we're trying to get laid!
This doesn't mean we don't also want a meaningful reationship. It just means we value sex tremendously. Yes I know sometimes what we do and how we act is completely counter productive to this idea but trust me, whatever it is we're doing or not doing has the intended result of getting laid.
Yes, we really do think about sex all the time. Yes it's ridiculous. We can't help ourselves. Stop being so beautiful and we'll stop wanting to have sex with you. When we first meet you we quickly think what you'd be like sexually. Even after we've had sex with you we think about the next time we're going to have sex with you. Will there even be a next time? Will it be better? Will it be worse? From our jobs, to our cars, to our clothes, everything is done with an eye toward catching your eye.
If you're feeling sick from hearing this or prefer to just think all men are fucking nuts then my advice is to date women or spend some quality time with a monk. We are who we are. Accept it. Embrace it.
A female friend of mine who I've known for over 20 years just told me "sex is no big deal." Yeah. To a woman it's no big deal! If you have a bottomless keg in your house with a beer tap and you can pour yourself a cold beer anytime you want...of course it's not going to be a big deal. Just pull the lever. Cha-Ching. If women want sex all they have to do is say so. Go ahead. Try it. Walk up to any random guy on the street or in the supermarket. You'll see.
On the other hand, men have to say they want it (a given) and then either work for it or pay for it...either way time and money are involved. No wonder we place a higher value on it than women do. Anything you can get whenever you want it for free with no strings attached is not something you could possibly value much. Sex to a man is huge deal! And to any man who tells you otherwise, he's full of shit and just trying to be cool and casual about it. Every time we have it, we think, "Damn! I just had sex!"
We're talking single men here. Married men at some point stopped worrying about it and have no choice but to accept the fact it's no longer a big deal. Trust me. I've spoken with many a married men about this. They argue how other things have become more important...their kids, their work, etc....Bullshit. They've just retired from all the bullshit that goes along with getting sex from their wives. So they inherently devalue its meaning. Yes sex drives decrease the older you get...but all that means is that maybe once a week is enough now...instead of daily. It's still vital....to men of all ages.
And I totally understand the notion of "meaningful sex" and how important that kind of sex is to women. Of course, guys want to have meaningful sex too. But short of meaningful sex any sex will do. It's all we think about. Even Crash Davis, Kevin Costner's character in Bull Durham, had to step out of the batter's box because he had that "Annie character" in his head. You are always in our thoughts. Always.
The meatier, more complicated answer to why we act the way we do is that men are in a constant state of proving our self worth. Often, our behavior has a very specific purpose. Unlike many women, men are not so random. We may be showcasing our unique abilities or fueling our ego. We need approval and we need to be loved...much more so than women.
Sex is the one thing we know to look to that can boost our confidence and manhood. That's the real reason we're so focused on getting laid....it's not just because we're horny animals...although we are...it's much more psychological with us than you ever thought possible. Whereas, many women want sex just because it feels good....Self esteem and feeling more "womanly" do not bear the same weight in sex as they do for men. Many of us can only feel "manly" when we're having sex.