N-O. Such a small word to have so many connotations surrounding it! In a tangent from a comment on the asking and giving post, let us examine some ways that we can handle this short, yet potent, word!
"When I ask a partner for something, how do I know I'm not asking for too much?" Ask any parent of a toddler, and they will be very clear that the use of the word "no" is one of the very first verbalizations that come on line, and it is used with a high degree of frequency. Why then, in the intervening years, do we forget how to use this word with those closest to us? If you ask for something that is too much for your partner to give, use of the word "no" is indicated. What if it isn't used? Well, the likely outcome is a partner that is doing something they don't want to do, for someone else's benefit. This may also be known as "martyring". See also "resentment", "angst" and "victim".
Why would someone do that to themselves? My suspicions include the idea that it comes back to social training, particularly around the piece that says we are supposed to be all things to our partner. This is the person you LOVE! Why would you say no to them? You are supposed to bend over backwards to "make them happy". Read that again, and tell me that this isn't something that might need some reprogramming. It's pervasive. Particularly for women, the desire to please is fairly ingrained and culturally reinforced, specifically with those we are emotionally bonded to.
When a request is made that is beyond our emotional or practical capacity, it can be hard to say no. We like to pretend that our capacity to support the needs of our partner(s) is infinite, but alas, we still have our own limitations, and saying no can be an explicit statement of that particular "failing". Still, when that is pulled out and examined by the light of day, it is obvious that the idea of having no boundaries is ridiculous!
It seems more palatable to have boundaries that are set up in advance than to acknowledge those limits in the moment. Let's say you have a boundary about meeting a potential metamour before things go to a sexual level. You haven't been able to coordinate schedules to mesh up yet, but your partner comes to you and says they would like to get your okay on sex for their date this evening. Do you: A) Waffle, waffle, waffle, well, if you really think that's what you want to do... B) Say "No, for now. Here are some times we could meet in the next few days so that we can move into that territory more comfortably." or C) Sure! Go for it. (Subtext: I'm going to make you suffer for this though!)?
Now, in theory, you have a simple boundary, and you think your partner is totally aware of this, so why are they making the request? The gall! It's easy to forget when someone nudges it that boundaries _do_ move, so perhaps your partner is just checking to see if you still feel this is an important line to hold.
Another reason a partner may be reluctant to give a "no" is the way that no's have been responded to in the past, either by you or another partner. If a no has lead to hellacious drama and gnashing of teeth, the incentive isn't there to be totally honest. Finding a way to give a "soft no", where the word never actually comes up, is usually sought at this point. Maybe it's a reminder that there was an agreed on boundary, or that the timing just isn't right, or that when certain conditions are met, there would be a yes in the offing, but there can be reluctance to be fully up front with a firm negative. Create a safe place within your relationship to disagree, and show your partner that you are willing to hear their no with respect, even if you still don't agree.
When I was a girl, going to church with my family, one of the questions that came up pretty regularly was "God didn't answer my prayers." My father would always reply the same way. "Sometimes, you get a yes, sometimes you get a no, sometimes you get a not right now. They are all answers." Just because the answer you get isn't the one you were hoping for doesn't make it bad information to have!
Another complication is the story telling one can do. Let's say you want to approach a partner with a request, but you think they'll say no. Many of us will come up with ways to avoid making the request because the outcome is considered a foregone conclusion. Unfortunately, there are times where our imaginations run farther afield than is useful! I can't even count the number of times where I ponied up a question, waiting for the axe to fall, and got an easy yes, or a quick fix to get one. Consensus is nice, but a simple yes or no can have more clarity.
Within the poly community there is a perception that having a definitive opinion is trying to control one's partner. If you are looking to give or deny permission, this is likely true. If you are sharing what your personal opinion is without ambiguity, not to force behavior, but to inform, this is what a healthy "no" is all about.