system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is being gay biologically possible?
gpt
1) Evolution does not eliminate everything that is not beneficial for reproduction. There are plenty of things about us that could potentially function better, or have no use at all, but which persist because they are not unforgivably detrimental to the species as a whole. 

2) A trait that evolves and appears to be non-useful for reproduction on its face, may simply be more subtly useful. I.e. non-parenting organisms can provide child-rearing assistance for parenting organisms, thus improving the level of care an offspring receives.

3) An apparently non-beneficial trait may simply be a side effect of a trait which actually is beneficial for reproduction. For example, some studies have suggested that female relatives of homosexuals have increased fecundity. The lack of reproduction from one party may be insufficient to offset the benefit of greater reproductive capacity in the other.

Basically, not everything is necessarily "progressive for evolution.' If an organism works enough of the time, it persists.