Much props to Kayumidome giving for background to Airi. So she was a slut up to this point. A lot of time in this type of a story, a character's MO like hers is inferred but never confirmed.
He is but they do deserve each other, and I would have said he deserves better if he didn't want to continue the affair. Imagine if you didn't know the thot you were fucking had a boyfriend and then he ended up, let's say, getting violent towards you when he found out. She's a nightcrawler.