For the sake of the man showing a misbegotten interest in spending the remainder of his waking years by your side, do him the favor of sharing your thoughts with him beforehand. I don't think anyone could engage in a lifelong commitment with someone that holds this level of benign indifference.
Are you actually serious about feeling this way, or is this just you being emotive and overly depressive in an online forum?
If after some brief self analysis you find your reasoning to resonate with the former, consider seeking professional help. I am serious about that.