Yah except when the mc had sex with Tiffany she was not yet ascended, she specifically told you that she was not on the pill, but was not worried, because all superhumans are infertile. At least as far as she knows .To be fair, you can explicitly impregnate a couple of monsters who specialize in breeding.
The infertility is explicitly explained as being caused by the fact that each individual superhuman is no longer human, but their own unique species with a population of 1. Tiffany is not a human, she is a Tiffany. etc
MC explicitly is a shapeshifter who can read DNA living creatures by touch and use that read DNA to assume their form.
Thus if anyone could get around that limitation, it would be the MC... well, him and/or Ella
That all said, I do agree it is a bit of a downer.
I haven't advanced much past that point so maybe other more knowledgeable characters contradict this.
Though it stands to reason that the mc should be able to alter his DNA, to be compatible, if it is not a matter of being infertile but rather compatibility.