"Because you are my husband! Maybe not here but in my time you were and I am a dutiful wife. I did not like hearing him besmirch your name or doubting how you were with Evie. Not with what I knew." It probably wouldn't matter much for him but that's what she believed now had to be her motivation. This Cormac wasn't her husband but she was married to Cormac McLaggen in the future and she was a good wife to him. She wasn't going to just let someone who really didn't know- or was too damn stubborn to admit any difference- besmirch his name. She wanted to uphold the McLaggen name and position.
"You may not care but I do. More then you probably realize because you cannot even conceive right now what we decide to do. Lavender can keep her mouth shut because she doesn't have with you what I did. I'm not saying her friendship is less but I have much more emotional investment."