These three things are true:
1) Harry Potter wants to father a baby, and he's found the perfect anonymous man to help (Candidate 222 in the male fertility program).
2) Every time Candidate 222 is chosen in the male fertility program, he is rejected once his identity as Severus Snape is revealed.
3) Snape pines for Potter but will never, ever, under any circumstances admit it. Because why would Potter ever want him back?