Straighter Than a Rainbow

Listen up, buttercup. This joke is easy peasy. Rainbows are known for being colorful and bent, kinda like the gays. So, when this person says they're straighter than a rainbow, they're saying they're super straight. Basically, they're saying the opposite of what you'd expect, which is kinda funny, right? Bet you didn't even know rainbows are bent, huh?