I guess I can understand his logic. He fucked a lizard not out of romantic interest but more so for well interest. Hes a reptile loving geek. Whereas having a sex with another human is something that he views differently, something that should be done with romantic interest, and to me this chapter cements that. Hes fine fucking her when shes in lizard form, but given how he has feelings for her, having sex with her when shes human is more intimate and is something that is more valued. So it makes sense when he has a sudden "outburst" when she recommends that.
Sure he isnt out right saying it, but we have clear indications of him liking her. Honestly his development in liking her is rather common so I aint worried.