In gay culture: a relationship in which one partner is the other partner's first real homosexual partner, usually defined by one experienced partner and one non-experienced partner. The experienced partner usually helps the other person become okay with being homosexual and comfortable in homosexual relationships. This relationship almost always fails.
Matt: I don't know why you're with Bary.
Loyd: Because I like him.
Matt: Yeah, but it's totally a gateway relationship.
Loyd: I know, I'm setting myself up for heartbreak.