In general, it's because we're hardwired to spread our seed around. That's, in simple evolutionary terms, what we're for. A man can commit to the most wonderful woman on earth, but if she ain't givin' him any, his eyes will start to wander.
Having a friendship first is a lovely romantic ideal in many people's eyes, but in practice it very rarely works. Most people categorise new people they meet as a 'yes, no' maybe'. Once in a category, it's hard to move out. Guys who get put in the friend category find it very hard to move into the lover category, which is why many prefer to start out as a lover and hope it moves onto something long term. If it doesn't, there's always the next.
As far as online dating goes, I had great success, back in the days when I was single. I had many women offer me sex in direct response to my profile (online flirting is an art!), before even meeting. I would expect sex on a first date, and would never go beyond 2 dates if it didn't happen. In my view, too many women want commitment before sex, but the world isn't like that any more. We're not all Victorian virgins desperate to hook up with the first person who says yes. Far better to find out if you're sexually compatible before making a commitment. Personally, I never had sex with women that I didn't think there was some chance of a long term thing happening, having already ascertained (via my profile) whether we had anything in common or not. The trick is to have a good profile, that screens out the crap. OK, so maybe there's always some dumbass who can't be bothered to read your profile, and just thinks 'hey a woman! I'll go for it!', but hopefully you possess the class and intelligence to screen them out before responding.
I have to say though, online dating might be good, but it sure doesn't beat real time meeting people. You just need to meet the right people at the right time in the right place. It can be tough, but it's SO worth it:-)
If you put an infinite number of monkeys in front of an infinite number of typewriters, eventually one will type "Hey, hey, we're the monkees".
Watch your spelling: the monkeys are catching up.
God must love idiots: he made so many.