Does 18th-century abolition cause 21st-century same-sex marriage?

In the Boston Globe, E. J. Graff reports that Massachusetts courts may rule in favor of same-sex marriage before July 13. If they do, then the first two states to allow same-sex marriage (or in Vermont's case, its legal equivalent) in the twenty-first century will be the first to have abolished slavery in the late eighteenth century.

The two states would also be making their decision in the same sequence. Vermont forbade slavery in its 1777 constitution, and a Massachusetts court decision abolished it in 1783, according to Kelley and Lewis's To Make Our World Anew: A History of African Americans. Why is this happening? (If it is happening. So far there are only one and a half data points.) My guess is that legal arguments for same-sex marriage are most likely to succeed in states that abolished slavery early and were therefore free to write (and rewrite, in the 1840s) their constitutions with strong and broad assertions about human rights.

If you look up 1790 in the Historical United States Census Data Browser, you'll see that at the time of the first national census, only Massachusetts (and its then-subsidiary Maine) and Vermont were absolutely free of slaves. Here are the states with the lowest proportion of slaves to total population in 1790:

• Vermont: 0%
  
• Massachusetts & Maine: 0%
  
• New Hampshire: 0.11%
  
• Rhode Island: 0.14%
  
• Pennsylvania: 0.85%
  
• Connecticut: 1.11%
  
• New York: 6.23%

In other words, if the pattern holds, don't hold your breath for same-sex marriage in New York, despite its ultraliberal reputation. Look for it next in Pennsylvania, which passsed the country's first abolition law in 1780, or in crusty, Republican New Hampshire, where slavery ended in 1783.