Similarly the birth of a child, not the agreement to try and conceive, is the bit that matters. (Although I'll accept the getting pregnant somewhat)

The act of marriage, of declaring in front of witnesses (friends/family) before the law that you bind yourself to another person in perpetuity is obviously the occasion that is important. Not when you decided to ask. Not when your partner said yes. When you do it.

(The party afterwards is sheer madness etc etc)

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.

I fundamentally disagree with this. I think that lots of people view this thing very differently, and to some extent that's just cultural and based on their own views and experiences.

(Basically, I'm going to go with agree to disagree because I think your view is valid albeit not one I ascribe to)

The day Bruce Wayne's parents died was the most important day of his life. It is most assuredly not the day he looks back on as the best day of his life.