I can certainly imagine having a special needs child is tough and could put a strain on even a strong relationship. And if the book was conveying that was a factor in Tina and Mark's troubled marriage it would likely make me more sympathetic towards Tina. But we aren't getting any clear explanation as to why things are bad other than she doesn't want to be there. I feel they either should be sitting down and talking about their problems and see if they can work through them (and not when your child is in the room and forced to listen to mom and dad fighting) or admit it's over and move on. Other than her job is very important to her, I just don't get Tina at all.
Sanford strikes me as smart and likely a very good surgeon, but perhaps very ill at ease in social situations. I work with lawyers rather than doctors, but his misunderstanding Monique at her parents house rang true to me. I guess our backgrounds are influencing how we see the characters. ;)