Well duh, our system requires room temperature; I just said that. What I meant was that reproductive organs used to be entirely internal, so there is likely some beneficial reason for sperm to have evolved that way, to require room temperature, to begin with. Which is what I don't know.

EDIT: Actually, TSoL, that might be the answer: this evolution would allow for having a system which produces both viable sperm and the deformed "buffer" sperm, which would (presumably) increase the likelihood of fertilization.