I agree - but I think Cancun was more significant than just 'going to Cancun'. Somehow, imv, it was a turning point - an end point after which she knew she would no longer be a part of Travis' life on any level... I don't know how, but I believe that somehow this was made abundantly clear.
She knew that losing Travis meant losing everything else:
The friends, the Mormon facade (and I mean NO disrespect to LDS folk, I just believe JA used her 'faith' falsely), making connections/staying with people with nicer houses... all of it... everything was connected to Travis, none of it was anything she 'brought' to the relationship (when there was one).
I think she expected to get her way that day... whatever that meant, and Travis wouldn't budge. That was unacceptable and just the idea made her so livid that she planned ahead just in case).
Either that or Travis had already turned the corner and she knew she was going to kill him no matter what - the sex being either just a sick 'so there!' on her part... to show him how much in control SHE was - in everything - ugh. She makes me ill.