Is it okay to be having sex with someone else's boyfriend if she treats him like shit and he loves you not her

I've been having sex with this guy since New Years and we've really just clicked you know like neither of us know what it is about each other it just feels real and the other day he told me he loved me, then the day after he told me that he has a girlfriend and said that he would leave her for me if it got to the point of that. I just want other people's opinions cause I can't talk to anyone else about it 😞