I read this in the mid-70's in my high school library. The story starts out on the alien planet, following the female lead through her day-to-day life, explaining the alien society and biology. Human astronauts land and begin interacting with the alien females, having sex with them, etc. This causes dramatic shifts in the aliens' breeding habits and eventually dooms them as a species. The book was pretty sexually explicit - especially for the mid-70's in a high school library. I'd be interested in re-reading it to find out if I'm remembering it as racier than it really was.
In "Brightness Falls From The Air" by James Tiptree, Jr. there are butterfly-like aliens, but humans do not actually have sex with them. Humans torture the aliens for the hormone-drug they secrete when hurt.