An expression taken from "buy now, pay later." This refers to the (often untrue) assumption that bisexual people are going through a phase and will eventually come out as gay. It is usually considered offensive.

A person who claims to be bi-sexual, but is really gay. This person will realize within a year or two that he or she is not attracted to the opposite sex at all.

When people come out as being Bi, before later revealing that they are actually Gay. Seen by some as easier than coming out.