I don't really think it matters. Yes, a WS who moves in with their AP CAN have reality hit them sooner, or they could be really good at ignoring things. The WS who really LOVES family, could still get a family "fix" by the children the AP has. There are single people who date and live together, and their are those that choose not to live together. Those relationships end all of the time.
Seriously, thinking about the WHY will just drive you crazy. There are so many reasons why and most of the reasoning takes place in a wayturd's head.
I know that this is you trying to understand why your husband isn't coming home. When you get into this kind of thinking, Atena, you really need to get yourself out of it for your own good. Sometimes, when I talk to my RL friends and I tell them about things that happened(like DS10 telling me that he thought WH and OW were fighting last Sunday), my friends say, "Isn't that a good sign Doesn't that mean it's crashing?" I respond with, "I know this sounds callous, but I don't care."
It is entertaining to me to hear about the arguments in the "perfect" world(I KNOW it's not perfect, PEOPLE aren't perfect). It doesn't change anything though. My WH ISN'T coming to me, with hat in hands saying, "I want to come home and work on this marriage." Until that happens, I don't CARE what happens with them. It doesn't make a difference in my marriage. The affair is still going on and I don't care how involved they are. They were already having SF and a "relationship" before WH left, all I did was take MYSELF out of it.