Opinions on being unfaithful?

Okay so... I've known this guy since I was 14 and he was 19... he always showed interest in me and we went out a couple of times but nothing serious, he never tried anything sexual. He always said" when you're 18 you'll be my girlfriend" fast forward 5 years later, Im 19, he's 24. He is in a "relationship" but we see each other like once or twice a month for sex, its been almost a year. His relationship is "open" but Im starting to feel like Im doing something very very wrong 😖 Im aware his girlfriend knows he meets me and he's aware his girlfriend sees other guys but still.... to be honest I just like the sex, he's the BEST, i dont feel anything for him though so is it even worth it?? I know I deserved to be judged 😭