Mainstream porn sites show a lot of weird practices (what's up with that strangulation fetish??) and I do think it has a bad influence.
I don't think age verification is a good solution, because we don't become immune to influence at age 18. Adults are just as vulnerable to copying poor behavior as minors.
I think we should do the opposite: Remove stigma associated with sexuality. Why can't more movies just include everyday sex scenes? Why do we need to make this distinction where you need to go to a different site if you want to see something more explicit than a nipple? Most people probably wouldn't even go to porn sites if they could just watch something steamy on Netflix.