Rob- Acknowledging your partner's fantasies without judgment is one thing, acting on all of them is another. Honesty in this area is of course key to a healthy R, and the ability to talk freely- but the dialog has to go both ways. I think that's where boundaries come in, and I learned that lesson the hard way. I let my H push me into going along with his sexual preferences and I got hurt in the process. I threw up the first time I went along with it, and it's gone downhill from there. (My first thread has a lot of details if you really want to know.) At this point, I'm not engaging in any sexual activity with H, and haven't for 2.5 months. I can't risk being physically intimate with him right now.
In summary- there are boundary issues when it comes to sexual fantasies.