The act of being bisexual. However, even though you like the opposite sex's fun stuff, you are still gay because you also enjoy the same sex's fun stuff.
Even though Johnny bangs mad chicks, he's bi-gay because he enjoys a penis here and there.
Bi-gay means that someone is bisexual but they are more into the same gender for sexual attraction and romance/relationships. Women who are like this are called bi-dykes or bykes.
John is a bi-gay man in that he prefers men for sex and relationships and he's just into sex/casual dating with women but he's not gay/homosexual since he's sexually attracted to women.
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.
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.