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.