Yeppers, that's a hard bridge to cross - but maybe now IS the good time. At 16 she's ready to start forming her own sexual identity and what she sees/experiences will become part of that. As parents, we want that identity to be a healthy one ! In most cases, more healthy than our own was
When we hit that point with our kids (we had boys & girls) the jist of the explanation went something like this.......
1> Sex and love are normal parts of being human
2> What you see portrayed in the public eye (media, exposed society etc) is often VERY different than reality.
3> The rule has always been that sex/love has been a private affair and therefore the 'truth' of this side of people seldom reaches the light. Nothing taking place in our love lives is new or perverted - only hidden. It is NOT that uncommon as you may think. And it's a personal choice.
4> It's in our nature as humans to care about many people. Restricting that too much can make you a cold, indifferent person. But HOW you choose to express that caring has to be balanced with knowledge and experience.
That's the general direction of discussion. The particular individuals involved will guide the depth of the discussion - as will timing.
It also can be good to provide some reference/reading material discussing alternatives to love & relationships. This generation of kids are not as naive as most of us were at that age. They are exposed to more than we think. Unfortunately much of that information is incomplete at best and total fallacy at worst. Give them CORRECT information - from reliable sources - and expect them to base their understanding and actions on that
And remind them that the pursuit of happiness - in a safe and kind/considerate way - is a big part of the human experience.