Out of character. Vic killing those close to him is nothing new, as seen by what he has done to the girl he loved and what he did to Stephen.
Vic going "i wants me some of dat milf invisi-pussy", however, is. He's Dr. Doom, not Dr. Light.
Although it can be argued that it's rape, and I would agree, the situation we are presented is completely detached from the reality of rape in our world. As such we should try to mitigate our emotional reactions to the word.
Doom isn't about "getting dat pussy", he's about having a family, and this one in particular. Even though it started with a memory loss, everything after was done entirely willingly by Sue. There is no mind control, there is no unwillingness, the story tells us that outside of that hypothetical event, there have been normal courtship, mating and relationship just like any other normal couple and family. She loves him unconditionally and he is grateful for that fact.
In fact, I would say that it's the ideal situation for Doom. Anything less, like mind control, wouldn't have been good enough for him. He would want Sue to genuinely fall in love with him, he would want to know that it could happen if Reed never were. And this is exactly what happened: as we are told, he met with a Reed-less Sue, and successfully court her and proceed to start a family. Whether she arrived in a life raft with her dad or she was transported from 616 and had her memory altered doesn't change the fact that after this very specific event, everything else happened naturally; as he would have wanted.
But yes, if Doom had altered her memories so that he can possess her, he is a terrible person and it's akin to rape in the sense that he got what she initially refused to give and would never agree to if she had an intact mind. But this is very,
very different from real-world rape and I don't think this word should be trivialised with such far-fetched stories.
PS: I don't know much about previous Doom stories, I'm basing my reflections entirely on what I read since the beginning of this event.