You absolutely need to be honest. Especially if you love this woman - which I'm guessing you do because you want to mary her. I'm not a professional psychologist but have been in the community for a long time and from what I see it is extremely difficult (if not impossible) to hold back your desires and needs. You are looking at a lifetime of ignoring those desires.
You owe it to the woman you love to be honest with her. It will come back to haunt you, her and any children that may come along if you are not honest now.
Typically, men that are in this situation end up marrying the woman and then leaving them in the long run for a man. I would advise you like the others--be honest. If she accepts it, then that is her choice, but you don't want to end up hurting anyone.