I agree it's not an absolute, immutable concept, but I think it's a hell of a lot clearer than "good" or "right" or "fair". What makes consent is the fact that either party can opt out at any time for any/no reason. It's consensual because we are decision-making agents, and in spite of privation we can still choose to refuse to transact. Consent is violated by force or direct threat of force, not by the circumstances the transacting parties may find themselves in.
If you're very horny can you still refuse to consent to sex? If you happen to be dying of thirst and I offer you a glass of water, can you refuse? You can. Are there may reasons why you would? I don't imagine so. Does it follow there there are no circumstances under which anyone would refuse? It does not.
Did anyone get my consent before pulling all the fish out of the ocean?
Did anyone get my consent before dumping toxic chemicals into the river that runs through my backyard?
At one point damage to the environment wasn’t considered a problem of the commons.
This hasn't always been the case, and still isn't everywhere today. Slavery still exists, so does misogyny.
I'm a fan of the saying "tolerance isn't a paradox, it's a peace treaty". To extend that, peace treaties are literally laws.
Obviously, not arguing for CP here, but I'm pointing out that the age of consent is indeed is a (valuable!) will imposed by society on others. I'm bringing CP up because it shows a bound on behavior which exists, and thus there is a line which needs to be drawn.
All I'm suggesting is that this liberal imposition - that relationships between adults (as above caveated) must be symmetrically consensual - should take precedence over how good, right, just, harmonious, or fair said relationships are perceived to be. If we think we can make something more fair but less consensual, we should not.