I was in an open relationship for a while, was a lot of fun, but we had ground rules regarding sex (protection, limits etc) however it was more the fact that some people knew we were open, and one or two of them assumed this meant we would sleep with literally anyone/everyone, and when they tried it on the GF, they came on a fair bit too strongly, and she freaked, had a bit of a breakdown and ended things with me (there's a whole extra backstory of her being a mental case but that's less important to the story)
Either way, I had some fun, she's now with one of the guys she was seeing at the time. You've just got to know what you want from the relationship and know what the other wants/expects.