You know, I got into a conversation about this with a good friend of mine once, and we were talking about successful relationships and how to make them work.
One of the things was the difference between love and lust (I know the OP said sex, but lust qualifies). He said if you want to know the difference between love and lust, ask yourself just one question, and answer it honestly..........................
Do I truly enjoy the person, or, do I enjoy MYSELF through the instrumentality of that person? If I enjoy the person, I'm there no matter what, job or no job, rich or poor, I just enjoy the person and that is why I am there.
If I enjoy myself through the instrumentality of their money, what happens if they go broke? Most likely, I'll leave. If I enjoy myself through the instrumentality of their looks, what happens when they get old? If I enjoy myself through the instrumentality of their popularity, what happens when it goes down?
And, remember, a relationship can survive on a ratio of 51 percent love, and 49 percent lust. If the ratios hit 50/50 or go 49 percent love and 51 percent lust, that is the point at where it starts to have problems. Why is the lust even included? Because face it...............there had to be something about the person that attracted you in the first place.