Baby, dear. Don’t say that. I know how you feel, but you have to think this through.
Right now, maybe, you think that just being with him is enough. But when you’re already with him, sooner or later, you would want more. You will never be satisfied with just calling him yours. You would also want his heart, his love, his attention. But that is something you can’t have. You know better than anyone that he loves someone else. So why do you need to torture yourself with more pain?
I’m never good at giving advices. But at least I could tell you this much, because I know how it feels. And I know how painful it was to be with someone who can’t love you back.