He also doesn't seem to realize that she doesn't love him (ntr route). I think that's what the NTR branch so far (i know its a tiny bit into it only) needs. He's in love with her, she "loves" him. He cant have sex for a while so she cheats (masturbation is totally a thing). And disproves all the love she lied about. You cant fully love somebody and cheat. She's using him. She might like him a bit but there is no love at all left.
The issue with a lot NTR games is the forced pretend love the girl supposedly still has for her boyfriend/husband. Or he still has for her once he finds out. Few games call out the girl, fewer let you leave her. Most you forgive and it still goes on and the ending says you're "happy".
Why are you focusing on the scene where she masturbates another being, and not noticing the circumstances that led to it? If this were a vanilla romantic story, then it would indeed be strange, but this is not a vanilla story at all. The circumstances are completely different, and the story leads to NTR, not romance. But that doesn't negate Mia's feelings for the main character.
Why did she do it? It's briefly described in the story, in her thoughts, in what she's thinking about. But if that's not enough, I can explain in more detail.
The Succubus Dungeon is a big expedition that they have all been preparing for a very long time, it is the peak of their shared history. After that, there is a slight decline, family, children, all the charms of family life. However, the dungeon is not easy, the succubi's mana raises libido to the skies. Throughout the week-long expedition, Mia saw how others relieved this burden with other creatures, but she could not allow herself to do so. When the tension reached its peak, she decided to take that risky but necessary step to release this monstrous tension from her body. Why did she need to do this? When you are weakened, you lose mana, and if Mia is not strong enough at the right moment, she may lose her beloved, unable to protect him. Therefore, it was critically important for her to do something about this situation at the time of the 0.4 update.