There is currently no way to allow only adult-verified avatars on a parcel except by having the parcel in an adult-content region. You have to choose "age verified" or "PIOF", either way you're excluding avatars you want to allow in. There are reasons other than the presence of adult content that people might want to restrict access by unverified avatars, and there is no intention that selecting this option imply that adult content is allowed on the parcel if it's not in an adult-content rated sim.