Hate to be the one raising this point, but since it hasn't been stated by anyone else..
Could she have been raped or molested...
FOr my personal experience it could explain some...
So here's my personal experience..
I was raped in my late teen (18 actually).. I avoided men for 6 months.. I was shell shocked.. Then I had sex with anything that moved.. I felt like a whore so why shouldn't I be treated like one.. I'm really concerned this could be an issue with your dd..
Also.. For her behavior.. I would have her sleep in a sleeping bag on your bedroom floor or move her bed into your room for a good month.. Install a bell on the outside of the door so she can't silence it when trying to sneak out (and I am sure she will) so it will wake you..
Then move her into her own room or a closer room with just her bed 2 sheets and a blanket, and her dresser.. Period.. That's it.. For another month..
All this time she is home.. Under supervision..
You need to explain to her the seriousness of her actions.. What she did was SERIOUSLY irresponsible.. So these are SERIOUSE consequences.. Explain your love for her.. And your concern.. Your fear over her actions..
On your dh.. You guys need a united front definately.. But YOU are her mother.. YOU are the one who needs to be establishing the rules, and as her step-father he needs to be backing YOUR decisions up.. Not the other way around.. She is your daughter.. He is simply her mother's husband.. KWIM.. Regardless of how fond he is of her, and she of him that is what it comes down to.. YOU love her unconditionally.. He may not.. At least in her mind..
Write down your expectations of her.. Clear.. Concise.. And the consequences for breaking those rules.. Also clear and concise.. That way your dh isn't left to his own reasoning about what you would consider appropriate punishment..
Best of Luck and Warm Squishy Feelings..