You don't have to apologize for your beliefs or explain your child raising. You can merely make it a "rule" you don't want your son dating until he's 15 (or whatever age you decide) just like one makes rules about when a child will get his/her driver's license. He'll know the age and can tell the other kids (and not invite girls out) and the parents will have to settle for "I don't feel Tom will be ready to date until age 15" or whatever.

Just make sure you don't comment on their parenting style either though. If they let their children date early, fine but your son isn't dating until X age or until he can drive himself and his date, etc., whtever you decide.