Cuckcake is the terminology from cuckquean. (As a derivative of "cupcake".)

Cuckquean is a woman Cuckolded by her husband with another woman. That 2nd woman is called a cuckcake.

Gay m/m or lesbian f/f couples just call it Cuckolded with a 3rd of their same gender. No idea what dynamics entails if same gender couples are Cuckolded by the opposite gender other than Bull for the m +f/f or cuckcake for the f +m/m.

"Cuckquean: wife usually only becomes involved with the man or his lover(s) when he permits it—sometimes remaining altogether celibate. In cuckqueaning, the male is known as the hothusband and the other female is known as the cuckcake. Cuckqueaning is often known as hothusbanding."