I think the way Alicia treats Peter is really shitty. She barely talks to him and hardly spends quality time with him. It's as though she feels that she's bestowing some great grace upon him by being his wife. Peter is also such a pushover in front of her.
I think if the roles were reversed and it was Peter who was always giving Alicia the cold shoulder it wouldn't have been tolerated. I know Peter slept around but he's really been well and truly apologetic about that and has not looked at another woman since. But after Will, she doesn't really have the right to use that as an eternal right to look down her nose at him.
I wouldn't be surprised if Peter had another affair. I'm not at all saying it would be okay for him to do so. But I do think that he gets no love at all from his own partner and whenever you have a marital situation devoid of any love and companionship then one of them will inevitably start looking elsewhere.
Do they even sleep together? They never do any real couple stuff like sit down and have a cup of tea together, or curl up in front of the tele, or sit in bed and have a conversation about something, or ring each other or text to say when he'll be home. They don't even tell each other they love each other.
They probably don't.