Nowhere in your post did mention love, although there was much mention of sex. While I have no issue with those who are poly sexual, don't get it confused with polyamory. In my experience, when genuine love is involved, as opposed to a more FWB scenario, people do not simply let others go on a whim. You equate your physical attractiveness with being able to attract more partners. And yes it does wane with age, and can be difficult if appearance is the basis for your relationships. You do realize that people - including monogamous people who lose a spouse through divorce or death - move on and find another mate whether they are in their 40's, 50's or even later. The relationships are based on far more than external appearance.
Forgive me for saying so, but what you describe sounds incredibly shallow.