A further point (and I wish I had a book suggestion for this):after we got past the "mechanics 'n'safety" part of the conversation, we steered away from "just don't" and also away from "don't be an aggressor" (which comes in many different varieties) and headed toward "enthusiastic consent", i.e. everyone involved should be enthusiastically (preferably verbally enthusiastically) on-board with whatever you're doing. (And be safe.)
I was pointed in this direction by a mom of a teenage girl, who commented "my high school sexual experiences were terrible, and that's not what I want for my daughter." And then I realized that "enthusiastic consent" was the standard I wanted the kid to be using.
Like many conversations, this one's easier to start before it's personal (i.e. before your kid is seriously looking at being sexy with someone), but you can refer back to it later.
Also, we've been carefully gender- and orientation- neutral, to the point where my younger son (after a comment about grandkids that started "you and your partner may someday decide...) said "Mom, I'm not there yet, but I'm pretty sure I'm going to be interested in girls."