There's things that are legal but morally wrong, just because somethings legal doesn't mean it's okay or right.
At 18 she is a teenager, she might be legally an adult but only just, you don't just turn 18 and suddenly become a mature adult. At 18 you are just learning what being an adult actually means, you are just learning really about proper adult sexual relationships. 18 is so young.
I am 22, I have nothing in common with 40 yr old men, we can have a chat and a laugh but I can't imagine we would ever be on the same level enough to have a sexual relationship, we just are at very different stages of life, and I can't imagine any decent 40 year old really wanting to be with me, I find that really creepy because I know how different we are.
Even now 18 year olds seem so young compared to me and I'm only 4 years older ffs! I don't know a single 18 year old who could be on the same level as a 39 yr old man (and I know plenty).
I have a few friends who have and are dating men in their 30s and every single one of these relationships have a power imbalance. They aren't two equal people, they are almost always "big strong man" teaching a "young girl" dynamic. I don't think big age gap relationships with people so young can ever be fair or equal.
It makes my skin crawl a bit, I know the type of man who hits on 18 year old girls in clubs, I look at 18 year olds and see a late teen, I don't see a fully formed adult ever, and that's before you even the sexual abuse into it.