Aren't we a little old for this now?
To solve the problem and put the endless chit chat about who can/can't/should/shouldn't commit to each other to rest I propose the lawyers create a side stepping movement legal documents that two people can buy and sign.
The wording is simple, 'Whilst I'm alive, I'll share my shit with you. When I die, you can have my shit.'
(some small print may be required to define 'shit')
Quick, easy, done.
If you need family and friends involved, then have a party that walks to the post box with the happy couple and watches the pop the FREEPOST envelope back to the solicitors.
If you need a faith involved then have a friend remind you that your particular God's love can only reside within you...etc...etc.
It's a simple solution that doesn't give a long run up to a flying fuck about what person you've chosen to stroke on the intimates once the party has gone home.
It's sort of like breakdown cover for a relationship.
You might be able to buy a My Shit/Your Shit Agreement at motorway services car park from guys in High-vis jackets in years to come...