This is one of the hardest things for me to navigate as a mama. Growing up it was so "hush, hush, shame, shame, bad, bad" that it is hard for me not to revert back to that on an reactionary level.
I'm not opposed to premarital sex, or sex outside of a long term committed relationship. I think it is ok for sex to be physical sometimes if that is what both partners hope to get out of it and nobody is being mislead. I really hope my kids wait until after they are 25 to choose a long term partner, but I don't expect them to wait that long to experience sexual contact.
I plan to tell my kids that some people do A, some do B and that they have to look within to figure out their truth. The only one who can figure out what they can live with internally (long term) is them. I think it is important to empower them and give them control over their own sexuality, because anything else is just perceived control. (IOW, they are the only ones who truly have control, so be honest about it.)
And of course, they will be well educated on birth control, STD's, and their body. I hope they will understand how the body works (aka natural family planning). My goal now is just to be calm, honest, open, and casual about the topic. So far so good.