Lori didn't treat Carol harshly because she knows everyone was stressed out.

Earlier in vol. 3, Carol hinted that her husband didn't treat her kindly, and later on it seemed that she had just lost Tyreese. Carol's a meek person, always looking for someone to comfort her. Maybe she thought with Tyreese (apparently) gone, Lori, who keeps Rick in check, would comfort her.

 

I was talking about Carol and Tyreese's relationship on the other thread and I remembered how bitchy and unstable she was. I think Lori always had a feeling that Carol was going to do something bad or hurt herself, so that's why he tried to explain why it won't work but Carol was already obsessed with these new world rules and threesome relationship that there was no turning back.