I am really identifying with your posts Prudence, and feel like I have a lot that I can learn from you and your partner's growth and revisiting of poly. I would seriously love to talk to you and pick your brain.... that aside.
I think that it's SO important that we respect our own boundaries and limitations, and that pacing be comfortable. I think it's okay to say no, and I too have experienced that non-verbal pressure to say yes that puts one in a corner of impossible decision making. Saying yes because that's the implied desire, even though it's not what you want is not healthy, and I think it's good that you spoke your truth and said no. I feel that if an agreement has been made that it needs to be respected - he can manage time better if having sex is a priority BEFORE it cuts into time that is meant to be spent with you/you both. I agree that the implication was that he was making one more important than the other by even asking, and I too would have struggled with that.
I think you're doing some pretty fuckin' inspiring work over there, and am impressed with your clarity, ability to look at yourself and the situation at hand, and challenge your own comfort zone. My question is this - if you are in a comfortable place, why challenge it right away? It's like growing out your hair forever, and just when it gets to your ideal length, cutting it short because you think you might like the way that it looks, then having to deal with years of growing it out again if you regret the haircut. Bad analogy maybe, but it has taken you a long time to get to this place, and in the grand scheme of things, a month or two seems like a good place to be curious about changing boundaries, but maybe unnecessary/premature. Just my take though!