I was just speaking in general terms, not about you. I was also just trying to present different opinions on a sensitive subject on which there is room for disagreement.
I guess I was just trying to caution you against assuming that the biomom's perceived indiscretions were to blame for your stepdaughter's problems. There are a lot of good parents who have had the red-face moment where they were caught by their kids in a compromising position with their spouse or SO. This can be particularly difficult for kids when they aren't familiar with the lover. It may be a problem or it may not be. Who knows?
There could have been some sort of sexual abuse by someone else you might never suspect. Another likelihood is that it may be a way of revolting against something that was not sexual at all, such as if her mother did not give her enough time with her father. Or a reaction to the divorce in general.
I would caution against over-reaction or playing the blame game. I do not believe that everything that goes wrong with kids must be the fault of the parent(s). I wouldn't assume that if a child started wetting the bed then they must have caught their parent peeing. It is probably an overt sign of some completely different emotional problem. Likewise you can't assume that any sexual problems must have been caused by some premature exposure to sexual situations.