I think you may have unconvered the very reason that they are getting married. Remember when Obamacare was thrust onto the public and San Fran Nan said it would have to pass for everyone to know what was in it? There were provisions for Congressional members and high ranking officials to be exempted from parts of it.
I think you have the right idea. There is obviously some benefit to be gained by getting married while in office. It has something to do with pensions and other benefits. I wonder if there has been some other legislation that has weakened DOMA in that regard.