"Heterosexual don't find the need to shout their sexuality from the roof tops, hold special rallies, or place banners on buses, then why do homosexuals."
Well, that's a very different question to the one of why gay marriage is justified, but I'll try to address it. I should probably say, I don't have much to do with it personally, but I'll try anyway.
The reason you have Pride rallies and things is largely historic/cultural. As you know, gay activism started in the 1960s in a campaign to correct various injustices and to attack homophobia generally. As you know, homosexuality was once illegal, and discrimination and homophobia used to be pretty rife. The gay community launched a campaign to correct those various injustices. The success of that campaign is something the gay community is very proud of, it forms an important part of the history and the culture which the LGBT community has in common, and all they really want to do is celebrate that. And you don't have to be gay to celebrate it - in the Pride parades I've been to, in each of the marches there were a huge number of heterosexuals who were equally interested in celebrating that history. The general attitude of these parades, if you've ever been to one, is one of inclusiveness and diversity. Everyone is welcome, and the unifying theme is the celebration of the shared history I've been talking about.
That's one reason you have Pride marches. The other reason is that gay rights organizations are extremely keen to raise awareness of parts of the world where those achievements haven't been made - particularly the Middle East. If you go to a Pride parade, there's often a huge amount of awareness raising about the plight of LGBT people abroad who are persecuted.
The reason for campaigns like the bus banners, I have to say, I hold less truck with - but again the idea behind it is awareness-raising. There are still homophobes/gay-bashers around in this country, and all those campaigns are there to do is challenge them and raise awareness that they still exist.