In this particular case, what rights and responsibilities did the assumed father actually have? He cannot, quite rightly, tell the pregnant woman what to do, whether that be abortion, adoption, anonymously give the baby up or keep it. So until she gives birth, his role in this pregnancy is a minor one.
Are you just dragging him into this in some sad attempt to lay blame elsewhere? Because we could also question why the mother led him to believe that he was the father in the first place. As this thread is full of assumptions, shall we also assume that the mother knew that the baby might not be his, and yet did not tell him that she had had sex with someone else? Such supposition is pointless, but I thought I might join in.
I feel sorry for her. I don't know the first thing about her, but it is an awful thing to have done, and I assume it will be on her conscience forever. The American judicial system is appallingly inhuman though, so this sentence is not really surprising.