I literally don't know what to think about it. I was always convinced that he:
considered needs like sex beneath his dignity. he always presented himself as a sort of God-figure, or at least a being much better than a mere man, so I assumed he was indifferent to most of common "man" temptations (GOF, ch.1 - on the contrary - if we look at the Greek Pantheon and the concept of anthropomorphism, him feeling and accepting lust, but still thinking about himself as a god/superman, is more understandable)
would see his child as a potential danger, especially when the child (in this case Delphi) had better genetic constitution than him (Bellatrix was a prodigious witch, Voldemort's father - an ordinary muggle). That means Delphi was not only the next Heir of Slytherin with toujours pur heritage from her mum's side, but theoretically, she could have much more magical power than Voldemort himself. So why take the risk and create recklessly a new unnecessary threat?
Or maybe he wasn't interested in making babies at all. And Delphi was a surprise.
But then, why did he let her live? I found this Q&A:
What exactly happened when Voldemort used the Avada Kedavra curse on Harry in the forest?
Again, Voldemort violated deep laws of magic he did not understand, but there is more to it than that. Having taken Harry’s blood into himself, Voldemort is keeping alive Lily’s protective power over Harry. So Voldemort himself acts almost like a Horcrux for Harry – except that the power of Lily’s sacrifice is a positive force that not only continues to tether Harry to life, but gives Voldemort himself one last chance (Dumbledore refers to this last hope in chapter 35). Voldemort has unwittingly put a few drops of goodness back inside himself; if he had repented, he could have been healed more deeply than anyone would have supposed. But, of course, he refused to feel remorse.
MV: Why was it important to you, Jo, to write about the cruelty and inhumanity?
JKR: I'm not sure why. But it was what I wanted to write about most. And it's about choice. And you are shown that Voldemort. Ummm... I suppose we're going to call him a psychopath. But he's so, in many ways, he is what he is and he's beyond redemption. Although this being Harry Potter and because I can take liberties because I have magic in my world, it is shown at the very end of the book that he did have a chance for redemption because he had taken into his body this-- this drop of hope or love--
Little love (but still) was put in Voldemort when he took Harry's blood at the Little Hangleton graveyard. He was given a tiny bit of humanity.
According to JKR, Lily's sacrifice gave Voldemort a chance to repent. I wonder if this small portion of goodness could have also prevented him from killing his unborn innocent child.