This thread has made me sign up after a few months of just visiting ^^
Anyways I'm a student in psychology and let me just say that there have been many books written on this subject as well as various ways of seeing it depending on if you're analyzing from a biological point of view, a evolutionary point of view etc...
A lot of the answers so far have had an element of truth in them at the very least, but our culture has deformed our views on sexuality so there are some distortions that I feel should be corrected. And some I should note were bang on in their theory (Finnboghi) and others were almost there (matrix3509,only part he got wrong was the population control part).
I think I might be posting a lot on this subject but for now I'll try and pace myself, I'll start with the evolutionary point of view:
-For men its been efficient to mate with as many mates as possible in order to increase the likely hood of offspring, back in the olden days this would be the best strategy for survival as often the males would be hunting and doing dangerous occuptions, putting them at risk of dying early. This isn't the case anymore but this where we evolved from, and where our instincts originate from. One reason nowadays why men fear sexual infedility more then they do emotional infidelity is because they might be raising the offspring of another, so not helping their own genes to propagate.
-For women it was in their best interests to be more selective with their mate, choosing the best mate available so they can pass on their genes to the offspring.
However having multiple mates was inneficient as the mates wouldn't help the woman in raising the offspring (see above) and so the best mating strategy was to stick to only 1 mate so that mate could help protect and care for the woman while she raised the child.
This is partly a reason why women fear emotional infidelity more then they do sexual infedelity. If it's emotional then the man might abandon the woman and take care of the other's offspring instead while if it's sexual the risk is lessened.
That'll be enough for now,just going to correct a few things before I end...
"Some men are like that. Other men are gentlemen.
It is just that there a so few gentlemen these days that men are stereotyped into being pigs who are hungry for sex."
No. That is a complete deformation. All men have the sexual drive and the desire in them.
There is no such thing as a gentlemen who's completely uninterested in sex. What you're trying to say is some can repress their animalistic instincts better then others.
You wouldn't be alive today if the desire to reproduce wasn't in you.
Kryson was very correct when he said "Men are genetically made to want to mate as much as possible. Guys who say this is not true are either A) lying, B) in denial, or C) truly asexual. I'd say A and B are the more likely answers. Wanting to have sex does not make one a pig and one can certainly be a "gentleman" and have a healthy lust for sex. The thing about humans is that we can easily go against our natural instincts if we so choose. It's just much easier to follow said instincts"
"Anyway, my obvious answer is that sex is great; I'm as obsessed about sex as I am about everything that makes me feel good: video games, an extremely one-sided argument in my favor, spinning around really fast until you get that brain-twisting high."
This is the opposite example, following your instincts. And following your instincts makes you feel good. That's why we follow them.
"But back to the topic at hand. Yes i believe men are obsessed with sex, its just Women tend to control it on a deeper level. We will ache your pole by not letting you stick it in our hole."
Indeed ^^ Women tend to master their own sexual urges much better then men do, and use it to their advantage in the mating game. Giving yourself up on the first night would be a "noobs" mistake as after that, your desired mate would likely lose interest in you if he has no emotional attachment.
"awh, but luvy remember our gender is also loaded with the quite stupid ones who are obsessed with the ones that will harm them, and often too weak to back away... but then again, I know many of my male friends have spent time with an utter soul killer who sucks the life out of them ( and NOT in the good way

)but they atleast get to the point where they can walk away from things that are not healthy."
The reason why women are typically attracted to the ones that would harm them are based on their archetypical idea of perfection. There is a reason why nice guys are less desirable then "bad boys" for the majority of women, at least at first.
Same goes for men, their archetype of the perfect woman usually favors physique over mental attributes, hence the "soul killer" you speak off

Again, at least at first.(I fear some will take offense at my comments :S)
Anyways that should be plenty enough for me and a rather long first post.
It's far from a complete resume (the dutch guy was right, cultural factors are VERY important) and there are a lot of things I've left out.
Please do not be offended by anything I have said, my tone was cut and dry simply because of the volume of information, and I have but grazed the subject here.
I spent a whole trimester on this subject and will be continuing to study it for 2 more years so trust me when I say human sexuality is not as simple as to be contained in a few easy fit all theories.