There is a vast gap between "yes, I'll make you a sandwich" and "here is a photo of genital herpes... please try and remember these lesions when your boyfriend mounts you later."

I just went through the EXACT same thing! I have disengaged and made it clear to SD17 and DH that I would no longer transport her anywhere, pay for anything extra (its her senior year and DH has been unemployed for about 6 months)etc. At first this didn't sit very well and DH did not take me seriously, anotherwords went with the norm, me handling ALL the kids. HOWEVER, I politely reminded him about the new rules, that I will no longer be responsible for SD17 or her outcomes, that SD17 is his responsibility. So there was some resistance at first, I would say the first couple weeks, but after that my DH picked right up in the responsibility department. My therapsit told me he had it in him all along, BUT if we dont relinquish the role, then DH will never do it or even try. Makes sense when you think about it. We have three kids total in the house, so I take care of the other two but not SD, NO WAY..NOT ANYMORE!!! Suprisingly this has started to run its course and after DH started having to take her everywhere, anotherwords it DIRECTLY affected him, now he is telling her to find rides, be responsible, pay for your own shit etc.

I know how hard this could be. I've been dealing with my SS14 for 4 years and it's a roller coaster ride. I cook, clean, drive, help with homework, etc... and get nothing in return but pure torture.
I get frustrated and angry with him but in the end he's been taught to be this way. He's in the early teen years and I know that things will only get worse from here on out. To keep a sain mind I continue to do what I can and do not expect anything in return so I'm not hurt or disappointed. Children can be very selfish and of course being the SM a lot of their problems are our fault.
I feel that as long as I'm doing whats right, even if it never appreciated, I can go to sleep at night and not worry about whether or not I should have done more, because in the end no one can ever say that I never did _____.
We have full custody of him and he sees his mother every other weekend and maybe, if she's up to it, she'll pick him up during the week to take him out to dinner. She's in and out of his life, IMO I think she is BPD and so is he (SS14), but I'm not doctor I'm only human and there is no normal person in this world who acts like they do.
Hang in there and do the best you can...Your priority is to keep the relationship between DH and yourself the best possible.
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Stand up strait, take a deep breath, exhale and smile

As far as the sex thing is concerned..you may have possibly overstepped. I don't disagree though with informing her about what may happen though if she does have sex. There's some scary crap out there that not curable. I had to tell my ex's daughter about sex because no one wanted to do it. It was an "ignore and it'll never happen" type situation, and you can't ignore sex and teenagers. They learn to much stupid crap from people their own age and may get seriously misinformed about things that can seriously affect their health and maybe end their life.

Sometimes it does take a village to raise a kid. You can't depend on the school to raise your child...and that's what a lot of people do! They want health class to explain to their children all about the birds and bees so they don't have to embarrass themselves around their kid. Bad idea, in my opinion.

"In three words I can sum up everything I've learned about life: It goes on." ~Robert Frost

I agree that as a custodial stepmother u cannot be a friend to yur SD...being in the field of law, I have seen adults "go down" because they knew something about a minor and didn't do anything about it....They have enough friends, imo, they need parents and adults who give them consequences for their irresponsible actions. Even if u disengage, which I think is fine and have done myself in the past, I would never advocate becoming a "friend".....u will be the loser in the long run..."prayerhelps knew, but did nothing about it"

A mother is not defined by the "b" or the "s" in front of her name, she is defined by how she handles the "mother" part.....

I used to listen to SD's 'secrets' but always told DH what she said. He assumed since her mom had total custody she was also aware of what her daughter was doing, which she was and allowed it. Well when SD started getting into trouble for acting out, both BM AND SD said that SD had told me so that DH should have been aware and stopped it, even though he is allowed zero authority or even access to talk with about such things, with his own child!

So I stopped listening to SD's secrets. I was going to tell her whatever she told me I would tell her dad, but didn't want to be the messenger either. So I just ignore any conversations that 'go there' and change the subject. It is a shame but SD and BM did it to themselves. They did this to me one too many times.
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Age cannot wither her, nor custom stale
Her infinite variety.

William Shakespeare, "Antony and Cleopatra", Act 2 scene 2