I agree with your point, to the extent that climbing a mountain or birthing a child have a clear end you can evaluate afterwards to mark if you did it or not.

I agree with your point, to the extent that climbing a mountain or birthing a child have a clear end you can evaluate afterwards to mark if you did it or not. Mariage in theory only has a beginning, and in our times it’s mainly a promise (signing the paper or vowing in front of costumed people doesn’t mean that much more anymore). Some people stay together without ever marrying, other sign the papers and do a wedding afterward (a lot of churches will actualy require the papers to be done before), or never. So, if the question is “where did it start ?”, my answer would be at the point where both people agreed to be together. At that point you are effectively together, already doing what you are saying.