^^^Well, I suppose in the cases of a one night stand and an extramarital affair, it would be possible to get their input. More likely than not, due to the inconvienence factor, they wouldn't contest the woman's choice to abort. If she chose to have the child, then their choices would pretty much remain what they are now. Incest and rape are thornier b/c of the illegal nature of the acts, let alone any moral questions.
In the case of who ultimately decides, that ultimately lies in the woman's purview b/c she could always induce a miscarriage if forced to carry a child to term. However, if she wants the child and he doesn't, he should be allowed to walk away since she has the sole right to have or kill the child. Regardless, if these people arrive at such an impasse, it's unlikely any relationship they have will survive very long as either party will likely deeply resent the other's decision.