Bellatrix just has weird taste in men x).
I think it would have been less weird if it was an adult child, born before Voldemort first defeat, and before he was this weird snaky being who has almost been dead for a long time. I'm not freaked by his relationshiped with Bellatrix because it was implied, but I still found the fact that he was still able to conceive a baby very strange. But yes, magic...