A poster like that could be a good place to begin to talk about some of the problems with young teens.
1 - If your body doesn't matter, why are the people in this poster modifying their bodies?
2 - Is it wearing certain clothes that makes someone a man or a woman?
3 - Our biological sex remains real and is important in many parts of our life, for example when we seek medical care, or want to have kids. What words are we meant to use to talk about our sex class for these purposes? Why are we being told it is wrong to talk about them? If we are mammals, why is it ok to talk about these words with dogs or gorillas but not human beings?
4 - If we are meant to love and care for people no matter what they look like, or what bodies they have, isn't that also true of ourselves? Should we not love, or at least try and get along with, the bodies we have? How is removing a healthy sex organ different from plastic surgery to change a nose you don't like? Is that not a problem of self-acceptance? How would we feel if someone wanted to change their skin colour?
Obviously there are a lot more things we could ask but I think something like that would be a good place to start for most young people, they'd be relatable questions Sex discrimination is important too, but it strikes me that it might be difficult for teens to relate to in terms of how our biology affects our experience with things like maternity leave etc.