In most developed countries there is no easy way for man to "opt out" of responsibility for an existing pregnancy. The argument goes that women can have an abortion or give the child up for adoption if the pregnancy is unwanted, but men have no guaranteed say in what happens. One slip up and they are on the hook for child support, etc.
Basically, some people think men should have a right to abdicate responsibility for a pregnancy at least up to a certain point.
Depend if society provide any social support to single parents. Here in Sweden I think there is a strong argument to say that children here are not worse off if they happen to have a single parent that get support from the state compared to a child that have single parent with support from the state and child support from a second parent. The point of the Swedish social welfare model is that all children has the same possibility in life regardless of the wealth of the parents, which mostly seems to be true for the less wealthy half of the population.
I'm not sure that's a constructive way to look at it. Arguably both adoption and abortion are harmful to the child. Arguably reducing taxes on a particular group is harmful to society at large. Both are done frequently, so I think there's probably more nuance than you imply.
Basically, some people think men should have a right to abdicate responsibility for a pregnancy at least up to a certain point.