Since there are a lot of sarcasm in this thread and in the forums in general, I do not know it this is a genuine question or not (I'm not the best at social cues, if you can tell...) But I'm going to answer it regardless. After scrutinising the riddle for a bit (I can't believe I did not guess it right away, the title was a giveaway), I think I've solved it. The answer is BAND
I'm made of rubber (rubber band), or march with brass;(Musical bands)
I hold back the hair of a fetching lass;(Hairbands)
Bilarro's were iron (this required a bit of web browsing, but the answer is the same); on a bride's hand, I flash.(wedding bands)
And what will you soon be, alas? (banned)
Oh, and if this is sarcasm...perhaps this would save other people a few seconds of their time trying to figure it out?