Your boundaries are things you yourself decide you will and will not do. Simple example: I do not want anyone to put anything in my ass. I will say no. I will enforce this.
What you are asking is, is it okay for this long-term couple to tell you what you can and can not do or say.
You can, and should, ask your gf about anything you need to know. Her husband doesn't get to decide what she does in her relationship with you. You don't get to decide what she does with her husband, right? It goes both ways.
Please find and read the Secondary's Bill of Rights. That will save us both a lot of time. Read it, reflect on it, share it with gf as a tool for deeper understanding of what's going on after just two months into a poly relationship. (I think this bill of rights is in the resource sticky at the top of this page. I will check.)
Is this her first foray into polyamory?