It seems to me that in the first generation, the proposals come like this: open the shed, work on your career...marriage proposal. If you reject, prepare a meal, eat, take a shower, marriage proposal. If you reject, it comes after about 2 work cycles.
The further you are in the game, the longer the span between proposals. The first one almost always comes just after the first work cycle is completed. After that, it seems to be random. Sometimes soon...sometimes a couple of years go by...
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If I can get them to think, I'm half way there...