Should I even marry him? (Rhetorical question)

If you love the guy, this is something you need to look past.
My wife had relations with a guy, turned out he was a cousin of mine. This was a few years before we met. She told me, we moved on. We see him from time to time, he is married, and we have never brought it up. It was in the past.
If you are dwelling on this matter, this hard, to think about not marrying him, then maybe this is the escuse you need to leave.

I think it's concerning that it's still on your mind. I'm not judging you for that--you feel the way you feel about it and that's legit, end of story--but I think you should at least make peace with the whole situation. If you have to go to counseling to work it out, do it, the same way you hit the gym when you need to stretch out a sore muscle. Your thoughts about this issue could be evidence of a deeper one in your head.

Marriages are tough under the best of circumstances; resolve this or it could be bad for your relationship.

Most of all, be honest with both yourself and your boyfriend. If the cousin thing is truly a dealbreaker for you, then it is, and you're not a bad person for feeling that way.

The probs will only increase,she’s not going to vanish overnight,yiu get it ???

Are you sure you even want to promise to marry him just now???

Clearly, you're still struggling with the weight of his past...

You need to bring this issue up with him. I also wonder, does the cousin seem interested still?

Perhaps this conversation should include whether or not he will include her moving forward. Explain how you feel and that you feel it’s a barrier for you.

You must be young. That sort of thing isn't important. You'll get over it.

The question is is he strong enough to take charge of you?

If that's going to be a cloud over head, absolutely not. It's the kind of friction a relationship doesn't need.

Geez give the poor dude a break. The past is the past. He doesnt talk about who you slept with previously does he?

Nobody can tell you if you should marry another person or not.
You have to decide for yourself if you can live with this ,if you can trust him, and if it'll cause you problems in the future.
And then you'll have to come to your conclusion, but think everything through first.