I think you should still thank your parents for their generous offer.
But tell them if you'd prefer to do things for yourself. Perhaps say that, once you find someone & have children (if that's your plan) that it would be a wonderful gift for his/her grandparents to make a gift to their RESP--education fund (or whatever you have where you are).
If your folks are looking for a house as an investment, well, they can do that w/o roping you into it.
