If someone wears a lot of rainbow stuff does this mean that he or she is gay?

Look, i wear the Rainbow all the time, what do you have against that?? I am not gay..

It's not the most reliable indicator in the world. The rainbow is sometimes used as a gay symbol, but not all rainbows or multi-coloured garments are deliberately intended to be a symbol of the wearer's sexuality.

If you do a google Image Search for "Rainbow Flag" (with or without quotation marks), you'll see the kind of rainbow that is generally used as a gay symbol. If the wearer is wearing arc rainbows with little fluffy clouds, or randomly multi-coloured clothing, it is probably not intended as commentary on their sexuality.

Of course, the rainbow flag looks very much like a standard representation of the spectrum, so it is possible that it does not represent the wearer's sexuality. It could be that the wearer simply likes the aesthetics of the colour progression, or is a Pink Floyd fan, or is unaware that a rainbow may be interepreted as a gay symbol.