Maybe you are the exception to the rule.

I realize I am writing from personal experience and not from a study, but in my not so short life I can count on one hand the couples who really accept a non-monogomous sexual relationship for a period of time more than about a year.

I've known very many (100 +/-) women as platonic friends, many (50 +/-) romantically and quite a few (20 +/-) sexually and none of the women I've known romantically or better and also none of my close women platonic friends see sex as something you can move into and out of in a relationship. Zero. Sex for them is always seen as a paradigm shift in a relationship.