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LW,
It is one of those strange truths in life: who a child is at 16 is not necessarily who she turns out to be. This girl still has a shot at the world, but should you marry her father without realizing your duty to her, you'll make her life much worse than it already is.
If you want the man you must take on the girl. If you don't want the girl, I would go so far as to say that you have a moral duty (as we all have a moral duty towards children) to leave the relationship. She trumps you here.