Ok. Ok. The implication is that the GEOMETRY of the situation did not work out. Assuming she's got a vagina where humans TYPICALLY have a vagina, and I have no doubts that she does, there is no reason tab A could not have entered slot B. Missionary, from the front, should totally have been possible, and they wouldn't be pronouncing the event a failure.
Okay.
My deduction is that she totally turned full-lizard, and he attempted to fuck from the side, as is her nature, but was unable to, because he doesn't have a penis that bends at that angle.
... Fellas, if your human partner turned into a full animal through magic or science or whatever else... and they are fully cognizant inside that body...
Does it count as bestiality??
With that question on the table he totally did state that he was attracted to (or at the least Loved) her full-lizard form 2 chapters ago, so.
Fucking manga.