I thought this was supposed to not happen (since you can't do it manually, and I do agree it's just rape), is this a bug or is this happening naturally if the slave initiates it intended?

It also just actually causes mechanical bugs too, as the slave gets assigned to the same double bed as the colonist, but they can't actually go to sleep there, so as soon as their rest need gets low, they just stand in one place and don't do anything until they collapse from exhaustion.