What we learn about him is the complete opposite. Mike seems to lie to his wife, and as a result they have to go to couples therapy as stated in Act II scene 1.
MIKE. (To Margaret.) See? What’d I tell you? My own wife thinks I’m a compulsive liar.
KATE. Well, it’s not like you don’t have a history.
MIKE. Hey, easy.
KATE. (Chuckles.) I’m sorry. That’ll come up in the next session. (Lindsay-Abaire 53)
When Margie decided to say that Joyce actually was mikes child he has a very immature reaction. Why did Margie decide to tell him thirty years later? Why did Margie lie to everyone for thirty years that she was premature? Was it for the satisfaction that she broke up with mike because he had a future for himself, or was she just being selfish? Margie wanted him to get out of Southie and make a life for himself. She thought that if she told him he would have stayed and he wouldn’t have a career he’d be stuck in Southie with her.
MARGARET. Joyce didn’t have all those …I just said she was premature so Gobie would think she / was his.
MIKE. You know this is bullshit, Katie.
…
MARGARET. Didn’t do that to you because you were going off to college! … BECAUSE I WAS