Yitay: R’ Moshe says that we assume that.
This reminds me of another shaila with my dumb friend http://www.theyeshivaworld.com/coffeeroom/topic/my-dumb-friend. So this friend thinks that he needs a girl who has some azus; because it fits his personality well. (He says this, again though, he is dumb.)
So the problem is that he can’t marry a BT, because if she has azus, then the assumption is she is a ben nidda. I told him that he is too dumb to know if she has azus anyway.