I've been with my partner for 10 years and we have been open since we started.
I think that relationships only work based on honesty, open or not. We were honest and open with each other about our feelings and thus, we learned things we didn't already know. My BF had a few bad closed relationships and wanted things open, I wanted it closed but mostly because I couldn't figure out the reasons why having it open was a good thing. I love him though, so I was willing to give it a chance as long as there were ground rules. Some of the rules we came up with are hilarious now, but back then, we didn't know how we felt about certain situations. There was a lot of fear and uncertainty involved.
It took me a while to get into the open relationship mentality. I feel rather fortunate that my BF is so understanding and patient as I was very much monogamous oriented and didn't want to sleep with anyone else. We started having threesomes and I guess that's where I began to loosen up but it still took me almost 5 years to feel comfortable enough to have sex without him and when I did, I totally fell for the guy…and this is where I discovered that I'm polyamorous minded. It was clear that a poly relationship was not something that was going to happen and eventually he moved away for a better job opportunity. Since then, It's happened to me 2 more times, both times in situations where things just doesn't fit. My BF and I have discussed the idea of polyamoury and have settled on that we aren't going to search for it, but if it happens, then it'll happen naturally.
In light of what I wrote above, I don't fall for every guy I've slept with, it's just that we view sex differently, he can pick up a random guy for sex and I just can't. I like knowing things about people and what makes them tick. he's able to have sex without any emotional attatchment, while I need the foreplay and intimacy to even get started.
My BF is a top and I'm versatile, so being open is usually the only time I get to top someone.
Over the years, we've shed almost all the rules we had, I trust him completely, I do get a bit jealous still but it's typically more when he hooks up with someone that I'm also attracted to..
Even though my BF fools around more than I do, I definately enjoy just knowing that I CAN hook up if I wanted to.