This does of course raise the question, is killing less of a crime than rape?
I think the way you worded the question invites the answer: You didn't ask "Is murder less of a crime than rape?"
In trying to stay away from politics, I'm going to assume that the vast majority of D&D players agree that it's acceptable for heroes to kill. In this sense, killing is different from murder by intent and need. Acceptable killing can be doing so to prevent a murder or other serious crime. Again, to avoid politics, I'm not going to drive too far down the rabbit hole of what "acceptable killing" is, but any group that doesn't have some threshold of acceptability for good-aligned PCs isn't going to have said PCs rise above 1st level.
Rape, like murder, would be evil. The non-evil version is sex, which (unlike killing) very few people would find objectionable in absolute terms (again, there are political land mines which shall be avoided). It could certainly be argued that rape is worse than murder, but that may be contextual (both have a range of violence that can be applied that doesn't warrant detailed discussion). Regardless, comparing evil acts and trying to rank them tends to be counter-productive, after a certain point -- they're all bad enough that decent people don't do them.
Anyway, the question really is "Is it acceptable to have a PC engage in these behaviors?"
I've played evil assassins. Those characters have sometimes ended up in groups with the stereotypical cackling cultist. There is a difference. The cackling cultists are disturbing when they truly revel in it. My assassin always had a sense that he wasn't an unlikable person, just a person with unlikable business, and that he could be reformed. Cackling cultists make for fine fantasy foes, but never the PCs, even in the anti-hero or sympathetic villain sense. Mengele would not be an acceptable choice for PC at my table.
Likewise, I've played characters that frequented prostitutes or had other sexual "issues". They certainly didn't go around raping people. I'm not opposed to having (non-graphic) rape as an implicit or explicit occurrence event, by a villain and off-camera. I'd certainly expect someone who'd experienced rape to have a different attitude, though. Unlike murder, you might actually have a rape victim sitting at your table, whether you know it or not, so handle with care. Honestly, I'd probably reserve explicit use of rape to the same sort of campaigns and scenarios in which I'd explicitly use murder of children -- which is rare.
If a player at my table presented the idea you're facing, I'd probably treat him the same way as if he said his goal was to ensure the propagation/dominance of the dragonborn race by either killing the young of other races or castrating males of any age. Both are fairly warped and not (IMO) the territory of PCs.