A gene, which possesed in a pair, makes one gay. If a person is only a carrier of the gay gene (meaning they only have one gay gene and one hetero gene) then the person is only subject to acting "gay" half of the time but does not sleep with the same sex, usually.
"That Dustin has the gay gene, I just saw him download a whole batch of gay midget cowboy porn."
Katie: "Yea, I know Dustin is married to a woman named Amanda but did you see how gay he always acts?"
Chris: "That is because he is a carrier of the gay gene."