Ooh AC your DH makes me cross, but probably because he shares some of the traits of my DP. You can't help it if you are thinking a lot about having children, it's not like you're purposefully refusing to stop thinking about it! And surely it's better for you to have some outlet (ie MN) to talk about those frustrations, if, like me, your partner resents 'having to talk about it all the time'.
WRT your agreement, surely part of any marriage/relationship is that you allow for the fact that your partner may change over time? It seems unrealistic to me to think you can know that you'll never want children any more than you could 'know' that you'll never want to quit the city and move to the country, or any other big life change. Couples counselling doesn't sound like too bad an idea in my opinion, at least it would guarantee you a time/place to discuss all this in full and work through it, rather than feeling (as I often do) that you are asking for a massive favour just to discuss your feelings and something that is important to you?
DP and I had a similar argument over the weekend; I'd been reading Baby Hunger (don't do it, it's terrifying) and so was thinking about the issues of how late we leave TTC and the fact that we're currently allowing work to take priority, which all my instincts are telling me is wrong-headed. He's at the point where work is taking up so much of his head/life/our house and time together 
that he just doesn't have space to think or talk about anything else. So he resents me trying to have conversations about TTC, and I resent his work! At least we've got it all out in the open now. And we're going away together for a month (hurrah) over Xmas so we'll finally have much needed space and time to talk properly.
My point is I suppose that if this is what we are thinking about a lot, then this is what we are thinking about and they need to help and support that thinking, even if it's a difficult for them, rather than requiring us to do some sort of bizarre and impossible mind control. MN is neither here nor there. Why is it OK to be 'obsessed' with all sorts of other things (football, work, F1, playstation etc) but if a woman is 'obsessed' about child-bearing then obviously she is irrational and hysterical, even though it's the single biggest thing which will impact your life, and therefore worthy of a lot of thought!? Grrrrrr!
Rant over, as you were.