No, you can't be in love with someone, while being physically involved with someone else.. that's literally the definition of cheating, unless you're polygamous, or in an open relationship, but you can't tell me his feelings run all that deep, if he rather sleep with some complete stranger he met a couple days ago, instead of actually TRYING to talk with her, or even attempting for it to work..
Also, between the flashback and the present not much time passed, if we look at his uniform, so it's possible that she also already had feelings for him at this point, but he literally was acting like he didn't care if we look at chapter 1, so i really hope they won't end up together.
He makes himself look so miserable and throws a pity party for himself, but he never really sought a conversation, or even tried to convey anything, instead he went and looked elsewhere, because it was easier.
