How and why would someone have sex with girl who is in a coma?

I mean I thought people only have sex with people who are conscious and say yes. I've heard of cases where men have raped a woman who is in a coma and I'm wondering would they just do whatever and stop when the person wakes up? Do they know when the person is going to wake up or something and time their rape so they don't get caught? What goes through the mind of a person who does this?