Sexual attraction is all in your brain. At least if you're a nematode.
Ricardipus has a great image if you want to see a nematode picture.
I always thought worms were hermaphrodites (both male and female) but the story, as usual, turns out to be a bit more complex. Researchers at the University of Utah have found that worms have definite preferences for one sex or the other. And, if they kill off certain kinds of cells, the preference for one sex or another can change.
From the University of Utah:
"They look like girls, but act and think like boys," says Jamie White, a postdoctoral researcher and first author of the new study. "The [same-sex attraction] behavior is part of the nervous system."
"We cannot say what this means for human sexual orientation, but it raises the possibility that sexual preference is wired in the brain," Jorgensen says. "Humans are subject to evolutionary forces just like worms. It seems possible that if sexual orientation is genetically wired in worms, it would be in people too. Humans have free will, so the picture is more complicated in people."
It's an interesting story and you can find the rest of it here at http://www.unews.utah.edu/p/?r=101207-1