Serious answer: it depends. There are many reasons many of us may be noisy during sex, some of which have been addressed:
Some guys like it. They encourage it, and it sticks.
Some of us are copying what we see in movies, porn or otherwise.
It makes some of us feel more turned on. Probably because of the porn association, actually.
It makes y’all come faster (most of the time). Sometimes, this is what we want.
Some of us just do it as it feels more and more intense 'cause it’s natural to vocalize a good feeling.
We’re getting rammed into at 483 miles per hour roughly four times per second. The diaphragm moves correspondingly and we go “uh!”. Purely physical, and not really related to performance. Just like getting tackled. Repeatdly.
Why DON’T guys make noise? Because women do. Men in movies (even most porn) don’t lose control and make funny noises (until the very end, and y’all are hysterical then!) Most of us learn how to act in all situations by watching people older than us. The only way most of us witness sex is movies. Since men are quiet in movies, men try to be quiet in the bedroom. Young men tend to make more noise than older men, probably because they haven’t yet learned control. Plus, there’s that whole counting baseball stats thing. You probably shouldn’t do that out loud unless you want to annoy your partner.
While some of my experience does come from Johnson & Bush, a regrettably large portion of my experience with orgasms comes from the research bureau of Johnson & Handy.
I suggest you hang around a while matey, you’ll get the feel of the board and posters eventually.
Having said that I wouldn’t advise you to cast doubts on the SDMB members ability to answer questions seriously, you could find yourself in the pit.
Oh wait!
I’m moderately noisy near orgasm because I found my partner likes the applause…
when I explained that hearing him make noise when he’s close to coming is a major turn-on for me, he quit being silent. MMMMMMM…
I’m a guy and I’m loud, especially towards the end. It feels waaay better to let it all out compared to holding it in.
I find porn women very comical. They even moan when nothing is happening! Suffice to say, I’ve never met a girl as loud as their digital counterparts…except towards the end, if it’s particularly good.