Would getting humans pregnant not require very finely tuned, precise DNA altering shapeshifting ? I mean down to the very cells and shit.
The sort of shapeshifting required to produce sperm that can impregnate humans would likely be the kind of thing the Mc isn't capable of yet. Probably it's akin to that convo he had with Xanthe, the Mc's advantage lies in his memory based abilities which could give him an edge over Ella, but she currently still has/had that very technical and precise shapeshifting capabilities which he hasn't reached yet.
What sperm a creature produces is coded into their DNA, and that kind of altering is beyond the Mc, atleast for now.