This means that you are 100% straight, but would let a man suck your dick, or a woman eat you out because it feels awesome!
Tim: Wanna be friends with benefits?
Rob: No dude, i'm straight.
Tim: OK? And? You could be biflexable.
Tim: You are straight but would let me suck your dick because it feels good.
Rob: I never thought of it that way. Ok, as long as i dont have to do anything to you.