I'm not sure about that. I have nothing against people who aren't into monogamy (as long as they aren't hurting anyone) but I love being in a monogamous relationship. I love just being with one person and I genuinely really hate the thought of cheating on him. Plus, there are actually some wild animals who stay with one mate for their whole life.

@58, First of all, theories are all over the map over what is the "natural" for human relationships: some point out the common nature of polygamous relationships in human history, others point out that humans resemble monogamous animals, with relatively modest sexual dimorphism (compared to Gorillas, etc) and relatively little evidence of extreme sexual selection (baboons oversized canines, etc). Ignoring that, you say that "by all accounts" marriage, and by extension monogamy, is an outdated social construct: that is a steaming pile of something unpleasant. The vast majority of people in the world and in any given country, and that includes the western world, believe in marriage, in one form or another, and many find it uplifting and enriching. In the US, where people point out the divorce rate, most people nevertheless keep looking for that partner even after divorce and live out the remainder of their life in that "outdated" institution of marriage and even those not engaging in marriage tend to seek out long term monogamous relationships. This is hardly the universal rejection of marriage or monogamy that you claim. Ignoring all of that, this FML isn't about her opinions on monogamy or marriage, it is about her being in what is implied to be an ostensibly monogamous relationship with OP while suddenly, as without any warning or discussion, saying she has major issues with that relationship type and then, also without solicitation, denying violation of that relationship. If you don't believe in or cannot handle being in a monogamous relationship, that is your concern. But it definitely sounds like she is currently or may be contemplating cheating. Even if she is just trying to work her way around to discussing an open relationship, this is still an FML for anyone who really does believe in monogamy. @OP, I don't know what to say. FYL.