I've been asked before about whether boundaries are appropriate in poly relationships and my answer ALWAYS is FUCK YES.
Of course boundaries are ok. In fact boundaries are vital. As in any relationship, but particularly in poly relationships, you need to be able to communicate with your partner what you are and are not comfortable with.
And you need to understand that this is a discussion, not a laying down of "You shalt not".
You will be bound by your partner's boundaries as much as you want to bind them by yours. Polyamory is a two way street.
The best way for me to talk about boundaries really is my own experience. James outed himself to me as bisexual a year after we got married. Remarkably this didn't really phase me. (I've since discovered that generally I'm in a minority of women who don't freak out at this type of announcement). So, James told me he was bi, and we worked out, over the years, an agreement as to how and when we could see sexual experience with men. There were rules/an agreement about when I had to be told, how he had to keep himself safe, etc. That was the boundary around that.
Then we decided to be more poly because James felt bad that despite my own bisexuality, I was not getting any other experience outside our marriage. So he told me that if I wanted to have sex with another man (because our other agreement was same sex is ok, opposite sex is not), then I should talk to him about it and we'd go from there.
Then later, when things still weren't working for me (reminder to self - don't pursue monogamous men), we talked about him having sex with a woman. We talked a lot, I was scared, insecure and frightened, but determined to do the right thing by James. That said I put 50 foot fences around the whole thing.
He talked to a nice female friend of ours that was interested in him, informed her about my boundaries and they agreed to have a sexual relationship. As I felt more secure in my relationship with James, I relaxed those boundaries... so gradually over time, it was no longer an issue to me. I knew that James wasn't going to leave me, that he could have sex with and love another woman and still love me and want to be married to me.
Today if James wants to enter into any kind of relationship with any other person, then all he has to do is tell me. As long as we can talk freely about who we are interested in and the other people we are in relationships with, then things are good between us.
And for those reading wondering whether or not I have any other partners, and if James is the only one having all the fun, I do have another partner. A lovely man and I'll probably talk about him in another post.