“Neither do many adults”.
IMHO, adults are adults. If they choose to have flings with as many people as they choose... then they are making an adult decision. A child is different. Yes, even a teen child. Even at a young age of 30, you knew that children were off limits. Didn’t matter how “hot” she looked... she was a child. There is the legal concerns of course. However, there are the moral issues as well.
We are increasingly cynical of 15 and 16 year old "youths" who riot and maim and murder... we don't tend to think of them as mere children when they get together as a gang and kick in some old man's head.
I'm not exactly sure if I can say I have a specific point here, honestly. I do know that you are right in that if this had been male-on-female, or male-on-male, I'd have a different take. I do disapprove of this woman (if she did it) but I'd want to know more before I locked her up in prison.