How?
The only girl she loved died, and she was pregnant to avoid being turned into a titan and leave that fate to her child. So, she got with someone that had nothing to do with all this war, she is now happy with a child + married giving/receiving the love that she never had before. And she is the QUEEN so people won't mess with her.
I can infer that she might be bi, but that is just my opinion.
But considering the times and circumstances, I don't think the Paradis people would allow her, as the Queen to be with another woman. And not only that, the technology is not advanced enough for her to consider artificial insemination to be able to have a child while with another woman (female). Since monarchy is a thing considering the social context of the story, she at least chose who she stayed with and manage to stay alive and well.
That is what I think for now.
It was alright but idc what anyone says, Historia should've had a better ending. WHAT THE FUCK WAS THAT ENDING FOR HER????