Marriage used to be defined by a distinctive act which is proper to marriage--- good old fashioned fertile intercourse, male and female, male part in female part, as recognized by every healthy mammal. Doing this consummated marriage. Doing this with somebody other than your spouse constituted adultery.
Now --- the Brits have noticed this --- the law can neither puzzle out what kind of jiggery-pokery shall be the constitutive act proper to marriage, not have any working definition of adultery.
It's ... it's whatever you say it is.
It's whatever you FEEL it is.
Feelz before Realz!