Dean is very much a part of their marriage, oddly enough. A lot of this is predicated on the things Dean and Sam learn about each other an d their relationship during the Month of Open Doors series. Sarah recognizes the odd closeness in them from the start, and she really is (as we see in Providence) both very intuitive and very frank with people. She's very much aware of how much Sam and Sam's happiness mean to Dean, and she also recognizes that rather wide streak of self-sacrifice he's got going. So she more or less consciously decides she's never going to give him a reason to have to practice it -- not on her account. And she loves Dean for her own reasons -- some relating to Sam and some not.