He was unwilling to raw-dog the dog.
Now the real question is... was it because:
A. He doesn't want to knock her up?
B. He doesn't want to risk STDs?
C. He is culturally conditioned not to do it raw?
Funny thing is, at least in most isekai, they actually explicitly state that beast races can't get knocked up by humans and vice versa (though in hentai series, humans can get knocked up by orcs, goblins, tentacle monsters, and basically everything else).
STDs are pretty much never even mentioned in these sorts of series. Unless there is some sort of magical reason why (like a Cure Disease spell that works on everything), such things should probably be common as muck in the average middle-ages fantasy setting.
SO I'm betting on C. He probably had sex ed classes that consisted primarily of
"Don't have sex ever. And if you do, always wear a condom. Because otherwise the world ends."
@Aguno
Sure, but that is treating pregnancy the same way the Final Destination films treat death. Technically it is possible, but the course of events required for it to happen is so convoluted that the odds of it happening naturally are silly low.
I mean consider that the walls of the womb are too acidic for sperm to survive contact with, so the only way for them to get through is either by coating the walls entirely with dead sperm to mitigate the problem (i.e. force of numbers) or perfect-gaming it by getting through without bumping into the walls (sperm don't have brains, nor sensory organs, so they tend to just go forward mindlessly).
PLUS of course if the timing is wrong for it then there is no ovum there, or no lining for the result to embed in.
Basically... just because something can happen in theory doesn't mean you'd ever witness it happening in practice. It just means it is possible. Like people who buy a lottery ticket every week of their life, and most aren't ever going to win anything.