Wow, I'm finally getting to the bottom of it.
A radical thought (to me, maybe others will go "DUH" when reading this): what if the best thing to do is to drop the the notion of equality/fairness altogether?
I've never questioned the feeling I've had since few years back regarding poly. That thought: why would somebody who is single want to start a relationship with me, when I am already married and cannot offer what a single person can? Is it even ethical of me to even start a relationship with another person when I am restricted in what I can offer her?
Well, universe showed me the first flaw it there: the person in question may be already married themselves. Well, that solves everything, because the "things I can't give" she already has satisfied in her marriage, right? Wrong. Her needs from me are pretty much a separate thing from her needs from him. A married person may want/need a lot to be in a relationship, and indeed I've found myself wanting a lot from her even as I'm already married. I've found that it has a lot more to do with the personalities of, and the dynamics between, the people involved.
However, there is a bigger flawed assumption that no doubt has to do with my difficulties in defining my own personal boundaries: that were I single I would have no constraints to "what I can offer" OR that the only constraints now come from being married to Alec. Thus, as long as I was monogamous with Alec, there were no limits to what I can offer to him, right? Oh, please. Just writing that down I can see it so completely not true.
It comes to something I'm still trying to learn to understand: a person can't meet all their partner's needs/wants. Funnily enough, since becoming poly I'm really flexible in what get from my partners as long as they treat me with love and respect. What is harder for me to internalise is that I am not responsible to provide everything my partner wants or needs.
There's the problem with aiming for equality or fairness; it includes entitlement, a right to something from me. When actually all my partner has a right to is what I have a right to from them: to be respected, loved, and to be taken into consideration. Not that those are small things, they are huge. But it is obvious to me that I treat anybody I care about like that.
Equality/fairness includes assumption of entitlement. It will no longer be my aim. When a partner of mine tells me they want or need something, I will make it a priority (again, not the but a priority), as I always do. I will try to focus on that more, and only worry about fairness if somebody else expresses that something is unfair to them.