I’ve been off active duty for more than a decade now, but I recall that a military installation and chapel were considered to be part of the state in which they were located in terms of civil legal issues. Therefore, when I would perform a wedding, I had to see that the couple had gotten a marriage license from a relevant local court.
In this case, that would be Maryland and whatever county includes West Point and its chapel. I have not heard that Maryland has passed a “gay marriage” law. Perhaps they have, and I have been unaware of it. In which case, the license for this “gay marriage” would have been duly obtained.
My other thought is that this lesbian party obtained its license in another state and is having the ceremony in Maryland. I’ve not heard that there is a federal marriage license, but perhaps there is one I don’t know about.
West Point is in New York, which does have gay marriage.
You’re thinking of the Naval Academy at Annapolis.