I've never understood this line of reasoning. I mean what adults discuss their sex lives with their parents? How many threads on mdc do you see that are about conflicts between ILs and parents butting in on topics such as religion, baptism, circumcision, vaccination, etc. etc.
Would you say that parents who don't want to tell their parents/ILs about their choice not to vaccinate are not mature enough to be having children? If I don't want to discuss my religion with my parents should I continue to follow their faith instead? Should gays be celibate until they come out of the closet to their parents?
You also don't know what the consequences are for these kids. For one of my friends, she was told she either had to marry him or be kicked out of the house. She married him, he was abusive, and leaving him became all the more complicated because they have a marriage license.
As to what is too young, it depends on the individual, but I would only allow it if it were legal (since my state has relatively reasonable guidelines about what's legal and what's not... in other states I might not have that requirement.)
if you're mature enough to make the decisions, you should be able to back them up. if i left my mother's church, i'd have no problem telling her. if i were gay, i'd STILL tell her. it's MY life...i'm an adult, i have a choice to make my decisions...AND the responsibility to take whatever consequences are wrought.
parents have the right to set reasonable limits on their children...i would personally not believe that to be who they'll be with for (hopefully) life. but if those were my only choices, i would have chosen celibacy, at least until i WAS out of his house.
i'm pretty open about sex, but i'm also pretty big on personal accountability. there's not much about sex that i consider taboo...other than adult/child relationships and incest. but if you're slinking in alleyways, you're not old enough, in my opinion.