I definitely think that's a big part of it. A lot of the people involved in these discussions probably have been sexually assaulted, but relatively few have had their lives affected by genocide (or even murder).

Puritanism probably plays a role, too. Even consensual sex is often considered an inappropriate topic, so it wouldn't be surprising if that applied doubly to rape.

Consider another explanation: If you use the term "rape culture", you essentially claim that in today's society, oftentimes rape will be accepted (FWIW, I agree with this statement). However, there appears to be a consensus in today's society that genocide is always a bad thing; generally that opinion is expressed without any qualifiers (as opposed to qualifiers like "legitimate rape" etc.)