Monogamy is the basic expectation in a relationship.

I´m not shure if monogamy is still the social basic expectation.
It´s surely my one, one of them. No debate, no negociation, neither a source of struggle about.
Agree with "is you job to remove yourself from relationships that are" not the ones you want.

It is but there has to be some sense of self security for instance if he is flirting and being sexual with everyone he meets then I highly agree but if he has female friends or business associates and they are on a friendly level then no that is more of a self and security problem not a relationship problem except for maybe that person needs to look Inward and trust their partner a little more till they give them reason not to

Not sure if the title of your post is always true. As you said yourself, it's not your job to change people's behavior.

Well said.

If you enter into a relationship and your partner is still enjoying multiple partners, then only you believe there is a relationship. It's a no-brainer.