I mean, Japanese stuff in general is going to usually push past what would be allowed in the West. But that isn't unique to Isekai. For example, Spice and Wolf is a fairly normal fantasy romance series, except the male MC is 25 and the girl appears to be 15 (of course she's a wolf goddess so she isn't actually 15, but it's specifically stated that she looks 15)

Obviously something like that just wouldn't fly in the west, but Japan is Japan, so it's normal. I could give plenty of other examples if you really want, but my point is just that your complaint has everything to do with Japan and nothing to do with Isekai specifically.

The American stories you mention appear to be self published erotica, so I'm not sure why you are expecting much out of them. Like no shit the main character goes around having sex with a bunch of hot girls, that's literally the premise.

And I don't think people from the modern era using guns against people armed with medieval levels of technology is equivalent to Japan's invasion of China in 1937. Like China was outmatched in that war, but they weren't THAT outmatched