Let him watch. It's his bathroom.

Anita:

[Director's cut scene. Anita goes to 54's ladies' room after fighting with Greg on Christmas Eve; Shane walks in behind her while she's at the sink]
You know, I told him before we got married, I told him what the deal was. I said, "I love you, but I need my freedom. I need to have fun, you know? Maybe not forever, but for now." And he said, "Okay, okay okay; whatever, you know, whatever you need. I want you to be free." My ass! I could kill him when he fucking gets like this, just kill him! [she turns back to Shane] Anita:
What's wrong? [Shane doesn't answer. Anita walks over to him, puts her hands on his shoulders, and looks at him for a moment. Then he kisses her passionately and they go into a nearby stall to have sex] Anita:
[notices Steve behind the stall door watching her and Shane have sex]
Shane. Shane, stop. Shane. Shane O'Shea:
[panting]
Let him watch. It's his bathroom. [Anita then hits him in the groin and runs out of the bathroom past Steve. Shane groans, retches a bit, and looks up to see Steve standing there looking at him. He slams the stall door shut, then leans back against the stall wall, bangs his head against it, and punches the stall door, immediately feeling guilty about what he's just done]