A natural progression.

I have been in a relationship with my boyfriend for four years. We've not lived in the same city for most of that time, I've has two distinct periods of mental illness, there's been family problems and deaths, we're both had periods of unemployment or being unhappy in our jobs. Despite that we've been happy. We hardly argue, we talk all the time, he really is my best friend. To be honest we act more like a lovey-dovey couple who have been together a few months.
However, in the past few months there has been a big change in our relationship. It's never been a jealous relationship, in fact it has never been explicitly monogamous and one of us checking out another person is by no means a big deal. In fact, it often leads to jokes about how my boyfriend has a 'type', it's very easy for me to tell in a room of people who will catch his attention, he can do the same about me. Early on in our relationship he made out with another girl, when my boyfriend told me he was really worried I'd break up with him, but I really didn't mind. I can get jealous, very occasionally, but was not at all in this case, it just isn't really something that happens to me.
Although this incident was not repeated I think it stuck with both of us. A few months ago I turned 26, marking 10 years since I had any kind of sexual or romantic contact with a woman. My boyfriend has always said he wouldn't want to stop me experiencing this side of my sexuality, but for some reason this began to seem more urgent to both of us. Our relationship was clearly not going anywhere, but neither was my desire to be with a woman. I had also said I wanted my first time having sex with a woman to be just me, not a threesome, which my boyfriend respected and understood. With some discussion we moved from an implicit agreement I could have sex with a woman to an explicit one.
I joined OkCupid, which has been a revelation. I met up with a couple of people, meetings that turned out to just be friendly but also made me feel a lot more confident about what I wanted. It was around that time that my boyfriend and I had a discussion about fairness, about how I could sleep with another person but he couldn't. We also talked about what would happen if I met someone I really liked, who I wanted more than friendship or sex with. I don't remember exactly how it happened, but suddenly it seemed like the obvious thing to open up our relationship, to stop being at all monogamous.
He also joined OkCupid, both our profiles are very honest about our non-monogamy, and discussing people we're talked to or met through it has become part of our regular conversations. I've also done a lot of reading about polyamoury and non-monogamy, the internet is excellent for this, and it turns out I know a surprising amount of people who are ethically non-monogamous. One of these is possibly my closest female friend, she has a husband and a boyfriend, and is a real inspiration to me. We've talked quite a lot about her non-monogamy, and mine, and it's been really interesting and helpful. In fact everyone who I have talked to about non-monogamy has been, I feel like I've learned so much in the last few weeks.
This is the part that is hard to write, because I am very aware of respecting other people's privacy. All I can say is that my boyfriend and I's non-monogamy is no longer theoretical, and the whole experience has been both better and easier than I was expecting. There is no jealousy or insecurity, and for everyone involved there has been good communication.
I understand that my boyfriend and I are at the start of this journey, and though it has been overwhelmingly positive so far there are likely to be bad times. Already things have happened to change the dynamics we thought we would have - I mentioned that there is an explicit agreement we can both sleep/ have relationships with other women (he's straight, I'm not). This was because he didn't think he would be able to cope with me having sex with another man, this has been tested to destruction and disproved.  So that's changed, in fact I have a date with a guy this week. It works because even before being non-monogamous we're very honest with each other and we communicate well.
I feel very positive about the way things are going, excited even. Non-monogamy is not for everyone, but I think it is for me, and I think it is for my boyfriend. I'm beginning to feel like a recent convert trying to get everyone else on board too, that's not what I'm doing, but I do want to talk about my experiences, and I'm just lucky they've been so good so far.