To springboard off of that.
When you love someone, it's more than just passion.
You become invested in their well-being entirely -- you want to spend time with them, you want them to do well, you want them to feel happy.
However, the truest test of love is how you act when that passion wanes.
Simply enjoying each other's company in silence can be a true mark of love. It doesn't have to be about sex, or kissing, or even hugging.
And sometimes, people fall out of love. Humans are not inherently monogamous creatures, though we have adapted to that lifestyle fairly well. It happens to even the truest of lovers. What's most important is what happens afterwards. Do you try to reforge the love? Or do you agree to move on and remain friends? Or do you pretend that nothing is wrong while your relationship decays from the inside out?
Also, sex and love are not the same thing. It seems obvious, but you'd be surprised at how often one is confused for the other.