The guy's involvement in a pregnancy can be as little as just having provided the seed - an enjoyable experience by all accounts.
The woman's involvement in a pregnancy is somewhat bigger. She has way more to gain or lose by keeping or aborting the baby. As such, she should have the final say on whether or not to keep the baby.
And sure, a guy's involvement / emotional investment
can be larger, but that's something very hard to quantify and thus hard to legislate. If you're in a stable relationship with someone, you should keep your husbands wishes concerning an abortion in account. But government can't (and shouldn't want) to regulate such a personal and grey area. Just as government doesn't legislate in which cases you're allowed to "cheat" on your boyfriend.
What I personally find more interesting is whether "
male abortion" should be allowed; men currently don't have any say in whether the child is kept, but they are forced to pay child support even if they decide at the first day of pregnancy that they don't want the child. Seems unfair to me.