This is a sentence is used by people who are trying to tell other people that they are not gay, but they are gay. So if you say this people automatically going to assume that you like the same gender as you are.
When two men, having a mutual respect for each other, give one another a hug and pat on the back, usually, three times in rapid succession. "pat... pat... pat" which translates to "im... not... gay"
I hadn't seen my friend in so long i hugged him, but i didn't want to seem gay so i have him the "Im not gay" hug.
"TRUST ME MOM IM NOT GAY UGH YOU NEVER BELIVE ME!" this phrase shows you are telling your mom you are nothomosexual. Im not gay mom means you are telling your mom that you are not homosexual.