I agree that it is not necessarily equal or fair for women to be able to "choose" after conception if men are just forced to live with their decision. But I would argue that, no matter what, there will be lots of unfair outcomes in these sort of circumstances. Here are the possible outcomes now, presuming abortion is legal in the situation:
1) Man and woman want the kid. The kid gets born and there is no disagreement.
2) Man and woman don't want the kid. An abortion is arranged and there is no disagreement. Alternatively, an adoption is arranged.
3) Woman wants the kid and the man doesn't. The man has to support a child that he did not want.
4) Woman does not want the kid and man does. Abortion happens and man is disappointed, but didn't do anything that he considers immoral.
In a world where men can "opt-out" of supporting a child during pregnancy, you only replace situation #3 with a world where a woman must choose to have an abortion, give a child up for adoption, or raise a child with no legal claim whatsoever to a father's support. I think that's much more unfair than #3.
I don't want to argue that men should be "punished" for sex by raising a child. I do want to argue that the world is better off enforcing a male obligation to support a child than forcing a woman to make a very difficult choice.
This is a very messy situation and there aren't any fair answers. All that I can say about this is the following:
1) It is best to be responsible about sex so that crap like this doesn't come up. No moralizing necessary: all situations above except #1 are not good at all.
2) This is not an argument for or against the morality of abortion. Pro-choice and pro-life people alike can agree that abortion is not ever an ideal circumstance. Given the reality that women carry children within them, it's not acceptable for a man to be able to make that choice. It's not equal, but circumstances aren't either. No matter what happens, there can't be equality, so we may as well concede the fact and try to do what's best.
3) Even where people aren't morally concerned with abortion, there is a lot of social and internal guilt surrounding the practice. It's best that nobody has to feel that, so let's all be responsible, OK?
I sympathize with men that want the same choice that women have, but it's intrinsically impossible for them to make that choice without forcing an abortion on a woman. Assuming nobody favors that, I have to stand against "male reproductive freedom".