I'm sorry to say, but they're not dating. Sasha just roped MC into her group of sex friends. Right after the incident, her disfavored sex friend, Sanada-senpai (the one she turned down twice before the retcon), begged her to take him back as part of a revenge plot. She told him that sex with him wasn’t fun, that she stopped admiring him as a Senpai once she got to know him, and that she wasn’t looking to add more people to her rotation.
Then he said he’d have any kind of sex she wanted if she took him back, so she suggested they have threesomes. Sasha not only expects MC to suck it up and watch her have sex with other guys, but she also expects him to suck it up and participate while she’s having sex with them—specifically with the guy he’s jealous of. I can’t decide if this is better or worse than being stuck in the cuck chair.
I get that some people might need to cope after witnessing such a twisted retcon, but there seems to be no end to Hagu’s depravity.