It is expected people don't cheat on eachother.
Patty and Deb have been in a committed relationship for 20 years, but Patty has been flirting with Sue at work recently.
Patty wants to have a romantic relationship with Sue.
Patty asks Deb about entering into a polyamorous relationship and gets Deb's approval.
Patty tells Sue that she will leave Deb someday soon and pursues her romantically.
Patty and Sue begin dating and eventually become romantic partners.
Deb discovers Patty cheating and uses it against her in court during their divorce.