HA! Ha ha ha ha ha ha. That is hilarious. Sex farts. A++. Not a whole lot you can do, I don't think. Even something in the silent-but-deadly category is going to be hard to pass off as something other than a sex fart. Pretty much every available method of (non-sex) fart denial (blaming it on the dog, etc.) is going to be weird in a sexual setting (why would you invite your gassy dog into the boudoir?) and anyways are total dad moves. I would try and keep dad moves out your sex life, as a general rule.
In any case, trying to deny that you farted is only going to make it more obvious that you did. We can leave the great "he who smelt it dealt it/he who denied it supplied it" debate to the scholars and just agree that this stuff happens. That's part of what makes sex so intimate. Of course there is a great intimacy in putting the various tabs into the various slots, I mean duh. But to me, the vulnerability of exposing the parts and processes of your body that are considered culturally off-limits to another human being and trusting that they will be respectful of them is an even greater intimacy.
It's an act of trust for many people to get naked in front of someone else, to allow someone to see what they look and act like in their most unguarded moments. Sex is fucking, but it is also a release of control in the pursuit of pleasure. All kinds of things happen to us as we approach orgasm: our ability to feel pain is lessened, our gag reflex disappears, we are physically stronger. Physiologically, having sex changes us.
Sex requires trust, and inherent within that trust, for me, is the understanding that things will happen that would otherwise be considered gross. In fucking, you will encounter jizz, poop, blood, myriad secretions, sweat, spit, smells, hair, and yes, farts. Part of the sex bargain is that you are cool about that stuff. Which is a long-winded way of saying: just laugh it off. It's funny, and it's how human bodies work, and if this guy's willing to get hip-deep into yours, then he can deal with a toot.
As far as prevention goes, I don't really know what sorts of measures exist. I mean, I wouldn't eat a big sloppy burrito right before getting rowdy, but that seems like a gimme. I guess just whatever prevents regular farting? I have heard (FROM OTHER PEOPLE) that Gas-X and Beano are a boon to the naturally flatulent. Certain positions are more fart-inducing than others: knees up to the chest, for example, or doggy. But really? Farts happen. Like almost anything, if you don't make a big deal out it, odds are your partner won't either. It would be sad to stop doing it doggy style because you were worried about farts.