I don't know if you're a slut. I'm not even sure what that means. I never could figure out the line between being a slut and having a healthy sex life. If a line exists, it's a very fine one.
I'm more concerned with your emotional health. In one breath you say he'll never marry you, but in the next breath you say you love him and want to marry him. That makes me think your reasons for continuing to shag him are about more than good sex. Are you sleeping with him because you enjoy it, or do you think it will increase your chances of marrying him? If it's the latter, you're setting yourself up for heartbreak. You can't sleep your way into someone's heart.
I'm curious why you are certain he will never marry you. Did he say he wouldn't? I agree with you that the odds are heavily against it, and I don't want to give you false hope, but people do marry in spite of their parents.
There's nothing wrong with having sex with someone you care about, but to me this feels like throwing fuel on a fire that's already raging. If I were as certain as you that there's no future between you and this guy, I would just cut ties and move on. Continuing to sleep with him doesn't make you a slut, but it makes you more vulnerable to future heartache.
Thanks for the question.