Your argument is valid. However, how many monogamous marriage have you seen without the incidents of cheating?

And when you look deeply into most open marriages is often because one party stepped out first before they resorted to opening their marriage.

Also "till death do us part" isn't really the reason people are against monogamous because divorce have given us the liberty to check out anytime.

However, the concept that monogamy ties women to one man (their husband) while the men are allowed to have more than one woman is why monogamous relationship are failing.

We shouldn't be surprised, though. Our generation wants equality and we are getting rid of patriarchal culture that hinds that vision, including monogamy.