Actually I think science has already found some survival value in having bi and gay people in a population. Children have an advantage when they have gay relatives because those gay uncles and aunts continue to serve as protectors within the extended family. Heterosexual uncles and aunts tend to go off and start their own families.
I think you can say the same for bi parents: Should a parent be killed, the remaining parent can partner with more potential mates if they are bi. Two parents is always safer for the kids than one parent, even if they are the same sex.
I like that because it sounds like it makes sense.
It is a simple and elegant explanation and I am eager to find out how the science around that theory develops.