Personally I think it's cultural as well. Like here in Oz, where I live now in FNQ anyway and not so much down south where I'm originally from, 'bro' or "brother' is very common - especially from the indigenous population. With them it's always 'hello my brother' and the ladies are sister. I don't have a problem with it - like I said it appears to be part of the culture! It's also said in jest sometimes. Like no offence to my Kiwi relations across the pond but when we're taking the piss (pronounced 'puss' for full effect lol) out of them we use 'bro' or even 'cuzzy bro' with our funny, thick, imitative accent lol - as I'm sure they have specific catch cries when taking the mickey out of us! I only know one other FTM IRL and we have our transsexual history in common and we get along and hang out etc, but I don't feel any special kindred bond with him just because our stories are similar. I call most people 'mate' - in true Aussie style haha!
The only thing I really get stuck on is when people call me 'mister' - like it's awesome and I enjoy it, but I'm continually turning around to look for my Dad!