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1.
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.
guy: you're gay, right?
me: I'm bi.
guy: bi now, gay later

or

guy: What was she doing with another woman? I thought she had a boyfriend.
girl: she's on the bi now gay later plan.
by bluepens August 27, 2005
 
2.
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.
So Carly says she's into Chris, but she's totally bi now, gay later.
by paddle May 03, 2010
 
3.
When people come out as being Bi, before later revealing that they are actually Gay. Seen by some as easier than coming out.
"Jack told me he was Bi last night."
"Yeah, just the other half now."
"Yep...Bi now, gay later"
by Mr GGGGGGGGG May 27, 2009