It may not be normal but I don't think it's immoral so long as the woman isn't being coerced into it. You have to accept that 18 year old women have agency too, they're not fragile flowers who have no concept of consent.

Yes abuse of power is a thing. But that's always true. Two people of the same age could have more of an abuse of power going on because of financial reasons, or physical strength reasons, than two people who are aged differently.