But that's an absurd patriarchal notion being overlaid the cultures that we've had for 97% of human history where the entire community or tribe takes care of everyone within it. Sexual monogamy is a very, very new invention, even though pair-bonding is an ancient one. Monogamy has it's roots in patriarchy - because prior to that time 6-9K years go when it first began, lineage went through the mother and no-one much cared who the father was -because everyone in the community takes care of everyone else.

You don't need to seek advice from a relationship psychologist before opening up a previously mongamous relationship, but you do need to read up on all the ways that polyamory or any other form of ethical non-monogamy is very different relationship style in ways that have more to do with just the number of people you are sleeping with. Books like Opening Up by Tristan Taormino are one such resource.

Naturally, people should have the relationships that work for them and make them happy, but I agree with Savage that if sexual exclusivity is the very most important thing in your relationship, you're probably headed for divorce. It doesn't mean people can't choose that and have happy, healthy relationships, but if it's an obsession that is the primary metric that determines whether the relationship is worthy or functional, it's just bowing to modern Western convention.