Despite Darcy's sardonic comment to Caroline Bingley that a lady's imagination is very rapid at jumping to matrimony, he is probably aware himself that that IS the decision he is avoiding by not figuring out his feelings for Elizabeth.

I assume that he thought about the possibility, then dismissed it, then it would sneak up on him again. I attribute Darcy saying she'll understand Rosings better on future visits as coming during one of those intermittent entertainments of the idea. I similarly attribute his silences as times when he has talked himself out of it again, and is dissatisfied and displeased with the idea of NOT marrying Elizabeth, but feels he is doing them both a favour by not suggesting such a felicity to her, since he will never allow such a degradation to himself.