Well first off, there would be a lot more dangerous abortions as unwilling mothers with child turn to sketchy back-alley abortionists in order to not be forced to birth a child, this leads to more women seriously injured or dying from a botched abortion.
If a guy imposes his will to NOT abort, I damn well expect him to be the full father and for the woman to have the choice of not being in the child’s life. The man has NO right to demand the child exists if he doesn’t intend to support it.
Then again, there will also be problems with a man pressuring a woman to abort when the woman doesn’t want to. This may lead to more murders, both the man murdering the woman with child, and the woman murdering the man to live in peace.
I will agree though that today’s rules are not the optimal rules, because men often do not have a say in the matter nowadays. However it goes against one’s basic liberty to allow a man, or anyone for that case, to force their will on a (pregnant or not) women. It’s a very tricky situation. Personally I feel the best solution would be to allow two consenting adults to enter into a contract prior to sex, in which they decide how they will proceed in the case of an unwanted pregnancy, and that contract is the only thing that matters if the situation happened. But let’s face it, people hate contracts anyways, let alone having one for a one-night stand. And it wasn’t until this past century where we’ve been able to seriously control our reproduction and thus this issue has become more prevalent. So until any laws are put in place, the best one can do is assume the other person is lying about their birth control, and protect oneself as best as can be, if you don’t want a kid.