According to my views, being a guy I will condem this act not because there is an element of virginity or any social atrocity is involved but there are feelings involved. It's not at all good to play with someone's feelings ( have been a victim of the same in the past ) as it has an impact on the behavior and outlook of a person.
But as the question mentioned that after the guy and the girl slept with each other then the feeling got developed I would say that both the parties knew what they were indulging into (basically without feeling for each other) so it isn't a good act or it isn't fair that you victimize yourself (either of the party) because you knew there was a sexual attraction and feeling rather than love or emotional attachment.
But on the other hand if the girl knew her feelings before the sexual act towards the boy she should tell the guy clearly because again you're responsible for your acts and your consent is there in whatever you are doing. The REAL guys would definitely understand and appreciate your honesty and your openness towards them and would act accordingly but in case if you meet the so called fuckbois treat it as an experience and not a traumatic event because Life is a lesson,learn it. And yeah I agree that feelings are involved but thats how life is, like a wave graph you experiences ups too and the down too.