In my view, not everything is negotiable. Those things that are not negotiable - I tend to call those things "needs" - are what I use to set boundaries. Those things that are negotiable - I call them "wants" - are used in the give and take to complete whatever the contract is.
I work hard to separate my needs from my wants. The only thing that would make my needs unethical is if they were generally unethical (e.g., called for murder, inflicting intentional harm, etc.).
So, if my needs themselves aren't unethical with respect to setting boundaries, then it is how I handle those needs that might make them unethical. For example, if I hid them while acting upon them and used those needs as an excuse for breaking the rules of a particular agreement.
The trick to remain in your integrity is to communicate those needs as early as reasonable and set the relationship on that solid ground. Selfishness doesn't come into the picture - as if our respective needs are incompatible, the relationship never starts or develops beyond the initial exploration stage.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”