Why don't you allow underlining? You allow italics and bold.
To avoid any confusion and frustration on the site, underlining is reserved for links only.
Sorry Boss, I can't resist. Only underlining links is not avoiding "any" confusion and frustration on the site.
Lots of the stories are about confusion or frustration. And some readers are confused no matter what you do. Maybe some authors. No names, no pack drill.
I suspect the underlining restriction is only on the forum pages, and as such, it does make it easier to tell what is and isn't a link, instead of having to rely on the colours - which do change with being activated and how long the browser remembers it. Anyway, for a forum post I can't see what you may need to emphasis that can't be done with bold, italics, capitals, or a mixture of them - even the use of brackets can help with that.