Or even the habit/tradition of getting married full stop. This is also a tradition that has it's roots in religious heritage.

Many institutions including Universities, States & many others have their roots in a religious foundation. If you asked 200 years ago, they may have been considered completely inalienable from each other.

What happens if you take a Church & surrounds & remove God? You keep the building where a community of mixed age demographics but roughly stable across the long term meet weekly and talk about ways of living ethically or just hang out? People are welcome to join. Community activities are run there. There are a few people responsible for keeping the community together, caring for members in temporary or permanent need, spiritual well being of the members (can mean psychological well being if you're more comfortable with that). They do this full time & are committed to it. This religiony thing has ceremonies & traditions attached to it. Particularly those that are 'public' like weddings & funerals.

What has belief in God got to do with any of that?