Marriying a fuck buddy
What do you think about a boy and a girl meeting very young and all through grade school they were best friends. Then when they got old enough they started to hook up but just for fun. Kind of semi dated by age 18 but both were still messing with other people. Until eventually one day they were both in need of a roomate so they moved in together. Then from then on they just took the role and husband and wife naturally just from the living situation, but did not even realise it was happening. Then they didnt like to see eachother mess with other people because it was too easy to see living with eachother. They just naturally became family without any planning to do so just based on convenience. Both parties combined their money without splitting it. Eventually officially got married to eachother.
They were best friends that were there to help eachother out and were a team and one day just woke up and realised they were actually basically in a marriage without ever even realising it.