Why is "If you don't want a baby, keep your legs shut" an invalid argument against abortion?

I don't think either is a sensible argument and I support the two same things that you do. If one ultimately believes that sex is merely for procreation, then it makes sense to stay abstinent in order to avoid pregnancy, but I believe that consent to sex does not equal consent to pregnancy. If a woman becomes pregnant and doesn't want it, she can abort, and following this, if a man gets a woman pregnant and wants her to abort, he should be able to relinquish all legal or parental rights to the child.