I think it might help to think about a spectrum of risk.
some behaviour the young adults do is very risky. I would for example be very concerned if my young adult son was regularly doing heroin. The risk of drug addiction, and of it derailing his life would be high and I’d be very concerned.
all behaviour carries risks of some kind.
walking through a park late at night while drunk carries risks. Getting into a car with your mate who is drunk driving is probably more risky.
the levels of risk that people can tolerate in their lives varies. Personally I’m a believer in living my life and experiencing what I can while taking measures to reduce risk that don’t actually impact too much on my life.
when I was at uni I regularly walked home drunk through parks. People (men, usually) asked me if I wasn’t afraid of what might happen. Nothing ever happened. (actually, as a uni town it was surprisingly busy midnight-2am most Fridays and Saturdays).
young adults generally aren’t as bothered about risk as older people are. They want to get out there and experience the world and try things, and sometimes this means they do things that carry risks that older people wouldn’t take.
they want to try tobacco. They want to try drinking various types of alcohol. They want to try illegal drugs (and most of them do and decide it isn’t worth it). They want to try clubbing all night. Many of them want to have lots of sex with various people almost certainly none of whom their parents will approve of. Lots of them have unprotected sex which carries all sorts of risk.
the level of risk that you are prepared to accept is very very low compared to most people which is why you are getting the replies you are on here.
at 18, and particularly as your dh doesn’t agree with you, the only way you have to enforce him behaving in a way you want him to is by constantly nagging and guilt tripping him.
realistically his response to this is likely to be either lying to you and/or leaving and going back to halls.
young people do take more risks than older people. But if you are genuinely concerned for him you might be better off thinking about how to ameliorate these risks.
for example, my dc and I attended a martial arts class in their teen years. I do not worry about my DS in London because he has spent years training in self defence (rule 1 run away where possible) and is a black belt. He can handle himself.
what can you suggest to do to help your dc cope if he was attacked?