It's all about smell. Scientists found that women can smell when a man is horny -- the experiment was as simple as getting some men sexually aroused, collecting their sweat and having women smell it. The women's brains were being scanned by an MRI at the moment they smelled the sweat samples, and sure enough, the horny sweat made their brains light up.
This guy just made them vomit.
If you're a dude and reading that makes you feel anxious, there's more bad luck coming. Your date can probably also smell your fear, and so can the guy at the table next to you. This was discovered in another experiment that was conducted in a similar way. They collected sweat from people of both sexes as they watched either scary or funny movies. Women were able to successfully identify which was the fear-sweat and which was the "laughing at Rob Schneider" sweat. Men didn't do quite as well, but still were able to identify both male fear and female joy from bodily fluids alone.
It's caused by a class of chemical signals, unoriginally called chemosignals, that are found in human sweat, tears and possibly other fluids. The extent to which chemosignals affect humans in day-to-day life is still under debate: It's hard to measure this kind of thing accurately, because a lot of the time the influence of chemosignals is subconscious.
"Wait, why do I keep having flashes of gay pornography?"