It can be trouble. I think it was part of the reason my marriage just fell to sh1t. Basically, we had different ideas of what was supposed to happen. First, if you're gonna have an 'open' mariage like that, you need to agree on the ground rules. More, you need to abide by them.
Rule 1: Honesty. Total trust is required or it'll wreck everything. If there isn't trust, then it's just cheating.
Rule 2: Emotional detachment. It's just sex. Keep it that way. If the third is someone where feelings are involved, it'll fall apart in a heart beat.
Rule 3: keep it part of both of your sex lives. If you're gonna do it, make it a part of the sex life of BOTH of you. Not someone on the side or for you or her. Remember the primary relationship and use the sex to ADD to it, not replace part of it.
Rule 4: Leave no doubt that if one of you is uncool with it, you'll walk away in a second. If the sig other feels threatened, it's all over but the crying.
Here's where my experience with it fell apart.
Rule 1: She wasn't honest. I told her everytime I so much as chatted with another woman, and she certainly knew everytime I did anything, WAY in advance (no spontaneous sex) and made sure I had express permission. She didn't tel me she was even talking to someone until it was pretty much a done deal. Made me feel like she just wouldn't have told me had I objected.
Rule 2: I saw it as just sex. It was nice to be able to chase tail or knock boots with someone, but that's it. She always kind of looked for a boyfriend, someone for an emotional attachment, which of course SHOULD have been me. So when she met someone, it wasn't just sex, it was a threat to my marriage.
Rule 3: I always looked at it as something we'd do for kicks together. She saw it as a totally separate relationship that didn't need to involve me. Complete screw up there. She simple forgot the 'marriage' part of 'open marriage'.
Rule 4: I wasn't in charge of it. She never really asked much permission, and often didn't even tell me she had a plan until it was already in motion. For example she 'asked' to meet a guy she knew for a date. I didn't even know she was talking to this guy, much less already set up a date. On top of that, I wasn't invited at all- it was a totally separate thing for her, which pi$$ed me off badly. I didn't feel like I could say no. She would simply not tell me anymore rather than NOT do it. Trust was gone, and it pooched the whole deal.