"I don't want to make butterflied lamb anymore..."

She wanted him to know that she's no longer out to please him or anyone. Double meaning was, that just because her husband was sleeping with his wife, she wasn't going to sleep with him.

I've seen this movie so many times and that line finally dawned on me at the end! LOL! I took it to mean that she was making a break from her past and that included Sam and his wife and everything connected to what she used to do and whom she used to know.

Sam says in his little speech in the very first scene that as long as Pippa keeps making those butterflied lambs then they're ok or something like that and goes on to praise what a wonderful supportive wife she is. So in the phone call at the end of the film she's saying she's tired of that.

Remember she rejected him in the party in the mansion when they first speak. No affair.

Great answers here but I still have one doubt. When Pippa says on the phone “I won’t be making butterflied lamb anymore,” there’s a very brief visual flashback showing Pippa and Sam ALONE, with her cooking lamb. What is that scene supposed to mean? It looks like a date if it’s only the two of them. That’s why the doubt remains, that they may have had an affair.