My fiance has a few friends who he became close with from studying abroad about 6+ years ago.
About 4 years ago we were invited to one of their weddings but have not seem them since. Keep in touch via FB every once in a while when an old memory comes up, but not really that much.
About 1 year ago we were invited to anothers wedding, but since we were invited to two weddings on that date, we chose the other wedding of a closer friend.
Most likely will not see either couple again for a while unless it is at our wedding or at a wedding of a mutual friend (there is only 1 mutual friend left who isn't married).
I know we should never feel obligated, but I really don't want to be rude!