I can't think of forums off the top of my head for this, but I do know there are in person support groups for significant others.
As for forums, I think a general lgbt board for significant others would be the only alternative to this site. To be honest, there really isn't much out there that I have seen. I did a quick google search and a few came up, but I don't know if they are good or get any traffic. Apparently Laura's playground has one and that site is pretty well known. I can't really say anything about it either way though.