Let´s see the list of First Ladies.
All Presidents from Harding on have been married throughout their period of office.
Woodrow Wilson became a widower in August 1914, and remarried in office, December 1915. In the meantime, his daughter Margaret Wilson (then 28 and unmarried) was the last First Lady not the wife of the president.
Benjamin Harrison became a widower in October 1892, and remarried 3 years after leaving office.
Then we have Cleveland - a 47 year old bachelor elected President, who went on to marry his 21 year old ward.
Chester Arthur was widowed a year before taking office, and did not remarry.
Buchanan was a President never married, taking office at 65.
John Tyler was widowed in September 1842, and remarried in June 1844.
Martin van Buren had been widowed 18 years before taking office, and not remarried.
Andrew Jackson was widowed 3 months before taking office, and did not remarry.
Jefferson was a widower 19 years before taking office, and did not remarry.
So... who have been the last seriously likely Presidents who were eligible to marry? Vice Presidents, and opposing candidates?