That's an interesting thought experiment. I'd say it really depends on how the sex started and what they were both doing throughout. I'd say letting the guy penetrate is basically accepting the chance of having children, because realistically, that will end with an exchange of sperm.
What I said before, though, still stands. Whether or not somebody wants children, she do not have the right to take a child's natural right to life away. Something a woman could do if she doesn't want children is take contraceptives that will stop a child from being conceived. the type that kill an already conceived child, though, should be banned.