I agree with Jessica. The subject is "one" which is singular. The verb should be "is". "One" is modified by a preposition phrase, "in two marriages". If this is a question on a grammar test, I would use "is".
However, when speaking many people will say "are" because they just said "marriages" which is plural and everyone is starting to forget about "one"; and also because one in two is fifty percent and that is a large number of marriages.