@vincentius lmao.
Good to know you think it's rape to fuck a woman who is horny and wants sex.
This is also a fantasy world, not modern-day Japan or the U.S. Like sure, I suppose that if an American went to a fantasy isekai world and fucked an underage catgirl or something and then went back to the U.S., they could prosecute him - something that came about by the "Protect Act of 2003", where illicit sexual conduct abroad with someone under the age of 18 can be punished back in the U.S. (I am unaware if a similar law applies to the Japanese, though). The prosecution would also have to establish that these other-world girls are humans. Does a fox-girl or dog-girl qualify as human under U.S. law? My guess is no.
That said, a few things:
There's no such thing as being able to 'legally give consent' in places where those legal concepts don't exist. As far as we know, it's not considered illegal or immoral in this isekai world to fuck these girls. There's also no legal concept around "She was too turned on, therefore it's rape."
We also don't know how old these girls are. What we do know is that they look similar to girls he knew back on Earth (although the sister-look-a-like appears to be older than his actual sister, I believe). But as far as we can tell, it is not actually his sister or classmate, but something more like a parallel world? That is of course if this world is even real.
We must also consider the possibility than he is dreaming, or that in some way this fantasy world is an illusion and not "reality". Fucking a girl, even raping a girl, in your imagination or dreams is not a crime. Not to mention that rape fantasies (for both men and women) are incredibly common. Being "raped" is the No. 1 most-common fantasy for women, with something like 40% of women having those thoughts. Women's fantasy is having the man she desires, desire her SO MUCH that he can't control himself and must ravish her). (Gee, it almost sounds like in those fantasies, the male is in heat and not in control of his own actions!)
I should add that if the last paragraph sounds like an intriguing philosophy topic of discussion, interested parties should check out The Chronicles of Thomas Covenant, the Unbeliever, where a man with leprosy and much self-loathing is transported to a fantasy world. He believes he is dreaming or under some kind of delusion, especially as he can travel back and forth between the two worlds (basically by falling asleep), and such is his disbelief that one of his first actions is to rape a 14-year-old girl who helps him. Much of the rest of the novel is spent on the ramifications of this action by a person who others in the world believe is a fated hero.