So I took some advice given here, and advanced the dialogue somewhat last night. I didn't mention that I had someone in mind. Baby steps!
I talked about what makes me uncomfortable about one night stands, and the reasons why I think doing this with someone I know already is actually less threatening to our relationship (I already know thier flaws and weaknesses and won't be as easily seduced by newness). I explained the various ways that I think this can make us a stronger couple who has lots more sex; I did my best to make it not just about me wanting to be slutty. It's not. That 'sperm competition" idea really appeals to me.
He was good, receptive, open. He said he doesn't want to stand in the way of new life experiences. He said all the right things.
But there was a sadness in his demeanor. I can tell he's not comfortable. As for his freedoms, he'd prefer to wait until I'm out of town. He clearly wants this stuff to be at a distance.
That sadness makes me feel really bad! What sorts of things can reassure a man of his importance and primacy in my life? How many ways can I describe that being open doesn't imply a deficiency between us?