1. A homosexual male.
2. A straight man who humps a homosexual man in the butt- not because he's gay, but because he's just desperate to get his rocks off. The Rainbow Rammer in this case would signify the "giver", not the "taker". The receiving person would be known as the "Rammee".
3. A far more colorful way to call someone stupid. (Simply replace the word "gay" with "rainbow rammer" in your day-to-day conversations)
Rainbow Ramming (ex: a gay fashion designer = a rainbow ramming fashion designer)
Rainbow Ram, (past tense: rainbow rammed)
This phrase came about because the widely accepted symbol for gay people is a rainbow, and, well, you can probably figure out the "rammer" part.
See also: fairy fucker
Taylor: Dude, last night I totally rainbow rammed that rainbow ramming idiot from my musical play!
Steve: You're gay now?
Taylor: Hell no! It's only gay if you kiss, or play the catcher!
Steve: Whatever, you rainbow rammer.