I'm not getting the problem with affirmative consent.
Consent requires a "yes." Assuming consent exists doesn't mean there is consent.
It's not rocket science. It simply means communicating clearly with someone else. Consent requires affirmation. Affirmative consent simply means that both people agree clearly that they want to do whatever they're doing. I can't imagine why anyone would want to continue without that, as a bare minimum.
Silence is not consent. Not resisting is not consent. Giving up and giving in is not consent. Consent is an affirmative thing. It is saying yes, either verbally or by taking some positive action that indicates a desire to continue what's going on. Lacking that, whatever is going on should not continue and certainly should not escalate.
My personal insistence on enthusiastic consent is a higher standard. It requires not only a simple approval, but active encouragement. That's only my own rule, though, and it's a rule I've always applied to any relationships I've had.
Some people think a written contract is needed. I think that's silly. People can, and should, communicate with each other, especially when having sex. That's what I recommend. Communicate and respect the other person's communications. Affirmative consent is consent. Anything less is not.
Seems simple enough to me.