Interesting observation. Never heard of the particular area having that situation. Sounds like Gretna Green, Scotland-type marriages for England & Wales couples, more liberal on the Scottish side for a long time (religious vs civil, age of consent, parental consents, etc). But for most of the 20th century I don't think Ontario was more liberal than Michigan, other than maybe age? There were historically strong family ties across the border in that area, much like Vermont/Quebec, could be that.