The Continental congress had to make law regarding what is a legitimate marriage, just as the government always does.
The first law regarding widow benefits at the federal level was in 1780. Naturally they had to have a definition of what they considered a legal marriage, and two men were not going to pass muster.
That’s more than a bit of a stretch. Sounds more like a pension law. Quote the part where they defined a legitimate marriage. I’m betting that it isn’t defined in the legislation because there was no reason to at the time.