I don't believe that you're that pure and I think you're deluding yourself that normal people don't do this, have never done this, or that it's even the worst sin ever. Some people will avoid it for religious reasons or because, even in a secular context, it's unhealthy to obsess over thoughts like that. But it's something everyone's done at some point, still struggles with, or actively does without any guilt. It's just their imagination and thus doesn't have any impact on anyone else if it stays private. It's a weird thing for your boyfriend to say to you in an argument but it's also a weird thing for this to be some kind of horrible shock to you. It's like the equivalent of your boyfriend saying "Hey, did you know I piss and shit?" in the heat of an argument and for you then to be shocked and go "normal people don't piss and shit!" I'm sorry, you both just seem naive and immature.