Depending on your view, the answer to that question might seem really obvious or very tricky and hazy.
At a recent conference I was part of in London, Alan McKee presented Healthy sexual development: a multidisciplinary framework for research. What McKee and his colleagues determined to be the core parts of healthy sexual development had me jumping up and down in my seat with joy (literally: I may have disturbed my fellow attendees with my bouncing). It summed up the things we try to support, encourage and inform our users with and keep core at Scarleteen so well, and so much of what I think -- after many years of thinking hard about and working with these issues, and being fully and broadly immersed in them with a very diverse population -- truly is central to healthy sexual development.
I'm delighted to have permission to excerpt and reprint this framework here.