A term used by mathematicians and logicians to indicate that something happens under very specific conditions. Often seen in proofs and theories.
1) The sum a geometric sequence is given by a/(1-r) if and only if |r| < 1.
2) In a bar:
dumped guy: "My girlfriend left me because I told her I loved her!"
other guy: "What a bitch!"
dumped guy: "No, it's all my fault, she's a mathematician, and I should have said 'I love you and only you'".
Some heterosexual men, in a two-male-one-female threesome, will avoid the sexual position "DP sandwich" because there is a good chance that one guy's dangling and swinging balls will rub against the the other guy's balls, creating gay pleasure and thus making the sex gay. The woman, to help the guys overcome their gay fears so that she can get what she wants, will encourage them by saying that the sex would only be gay if their balls touched.
Woman: C'mon guys, give it to me! At the same time! I'm so horny.
Man 1: No way, that's totally gay.
Man 2: Yeah, that's gay.
Woman: What is this, the Rocky & Bullwinkle show?! C'mon guys, think about it, it's only gay if your balls touch.
Man 1: (To Woman:) OK, maybe you're right. (To Man 2:) But I'm going to be on top because I don't trust you and your low-hanging balls.
Man 2: Sure, man, whatever.
Derivation: U.S. Navy slang. An entreaty used by a desperately horny male sailor to get another male sailor to engage in clandestine sexual activity for the purpose of mutual release. Based on the theory that when two guys on a ship with no access to females have sex, they are not gay, but just helping out a shipmate.
HT2 Betz: "Dude, if you jack me off, I'll jack you off."
AO2 Griffin: "No, way, man! That's gay!"
HT2 Betz: "We've been at sea for three months, and we won't see Pearl for another three. I'm just saying help a shipmate out. After all, it's only queer if you're tied to the pier."
AO2 Griffin: "Yeah, I guess you're right. We could go to the chain locker..."
A reasurance used toward to guys who are about to, or who you wish were about to, engage in homosexual activity. With just eachother, or with you as well.
It's not true, of course, as simply doing something with someone else can't change who you're attracted to, but with dudes being so afraid to be gay, it's worth a try.