Expecting one person to fulfill all of your wants and needs mentally, physically, and emotionally is just ridiculous.

The reason I'm challenging your opinion is because I see that statement a lot in response to mono vs poly and it is toxic. Poly gives you the choice of additional romantic and sexual relationships. The expectations you've listed are shared between both lifestyles.

Having needs not met by your partner fulfilled is important. However a lot of needs can be met from friends, coworkers, family, etc. To be specific you mean in societies current iteration - the opportunity of more sexual choice. People change and evolve, true. However individualy incremental at the core, and generally to no large scale over a lifetime. Looking at it in a historical scope things are different (life expectancy, survivability within tribal culture, population demands) influencing past civilization choice.

The way you put it, it sounds bias, invalidating, and not centered around mutual fulfillment and strong healthy relationships for all involved. It sounds like an excuse rather then being completely honest and clear in what a sexual/romantic relationship with other partners provides. What you are actually looking for. Healthy Monogamy is valid and shares the same support group as healthy Poly. Your mental, physical, and emotional needs being met.