How can it be ok to say to teenagers that group sex, gang bangs etc are acceptable?
Because they are, if all the participants are of legal age, fully consenting, and know what they are getting into and where their limits are.
Why can't we say that this behaviour is damaging?
"Can be" damaging, when the conditions above are not fully met. It's not inevitable that it will be.
I have never ever met a promiscuous female who doesn't have serious self esteem issues/substance abuse problems.
Are you in the habit of quizzing every female of your acquaintance about her sexual behaviour and number of partners? Because if not, how do you know you haven't met some perfectly happy, self-confident and well-adjusted "promiscuous" females?
The best message we can give a young person is that their sexual choices once they reach the age of consent are up to them and them alone, that there is nothing wrong in having whatever kind of sex they want, however frequently or infrequently they want it, so long as they and their partner(s) are fully consenting, they are sensible about protecting their and their partner's sexual health and they feel confident in saying "stop" if they reach a limit they aren't comfortable with. That if they want to wait for an emotional connection, that's fine. That if they want to experiment, that's fine. That if they want to have sex for fun, that's fine. That if they don't feel comfortable, they can say "no" or "stop" and that's fine. That they shouldn't feel obliged or pressured into doing anything they don't want to do, and that includes denying their own autonomy as a sexual human being for the sake of social conventions just as much as it does not performing some sex act their partner saw in a porn movie or doing X or Y because "everyone else is doing it".