Indeed. And as a heterosexual non-muslim, arguments that "Allah forbids it" will obviously hold no sway whatsoever over me. However, I would be open to rational secular arguments like you're suggesting, if there are any. I have yet to encounter any good ones.
The best argument I have against gay marriage would be against marriage itself. One may argue that people should not get special rights just because they find a spouse (of the opposite sex or otherwise) and that this discriminates against single people. One could also put forth a feminist argument, that marriage started as the ownership of women by men, etc.
But I'm aware of no good arguments that would support heterosexual marriage but oppose homosexual marriage. The only arguments I've seen for that boil down to tradition and/or bigotry dressed up as religion.