My personal feeling - speaking as a gay man and therefore somewhat personally linked, but also as somebody who has not once suffered bullying or mistreatment because of it - is that there are two crucial reasons gay marriage should be legalised, and neither are to do with "made up customs".
Firstly the legal side: inheritance law, hospital visitation rights, insurance, etc. etc.
Secondly, the impact it has on how people view homosexuality. A step towards equality is a step towards not seeing gay kids bullied, gay teens killing themselves, gay adults being harassed in the work place, etc. And that's a huge deal in my opinion.