Many women in the old days if they were not married by a certain age they probably never would be. Then there were the ones who may have married but were widowed, perhaps before having children and stayed single there after.
Also there were laws about marrying first cousins and such there might not be people around to marry who were not related. I have heard this can be a problem in the Huterites and Amish and Mennonite communities even today.
In the 18th century there were not laws against marrying cousins. Mr. Mercat and both have ancestors for eastern Kentucky, in fact two of our distant cousins married each other in October 1800. We joke that our ancestors looked around at the time of the Civil War, either didn’t like their cousins or were on different sides of the conflict and moved west. My great great grandparents were not related at the time of their marriage but they were from different sides of the conflict. At 18 they eloped and then moved to Missouri. I love genealogy.