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I remember reading somewhere that a logical conclusion, if considering the anime's canon, would be that humans are of the humanshape and monster egg groups. Humanshape is obvious, but considering Ash's Bayleef's attraction to him (and ruling out the notion of humans being of the grass group), he must have shared the monster group with her.
Personally though I don't think humans and Pokemon are biologically compatible to the point of being able to conceive children together. I would go into the prospect of sexual attraction being a matter of sentience/sapience among different species, but there's a whole screencap about that somewhere.
Anyone have that, while I'm at it? It's the one where all the posts have "I'm ok with this" variants attached.