It's honestly really funny how Japanese story never let the MC has sex if they come from Japan. The other isekai story the author makes, the MC (Makoto) never have any sex until the main story is canonically done, but once it's done he had sex with all the harem girls.
But since Eugene doesn't comes from Japan it's the girl Sumire who is from Japan, so sex is ok. And since Sumire is not the MC, sex is ok too for Sumire
(I actually forgot when it happened for Sumire. But Volume 3 of the LN has some very nsfw art for her)