But that's the point, they are not dating and she is not with MC. Eva makes that point multiple times. They are friends who occasionally have sex and she doesn't want anything more than that.
To me it came off as at least friends-with-benefits with a possibility of more down the line but she didn't want a hard commitment atm. Eva herself says "I propose we keep seeing each other like this and see where it takes us, this is while they are technically on a small date. None of that tells me she is still going to see others. If that were the case I'd at least like a little extra warning. Other games have done something similar where a girl doesn't want to commit atm just to have the girl eventually end up doing so. So, I don't see it as unlikely until told otherwise.