I think both of your points are important, but in an ideal world, I would go in a slightly different direction, and a bit further. First, I think that marriage is actually two distinct concepts - religious and legal. Since marriage was originally defined by religion, I don't see a problem if religion retains the right of defining marriage - a union between a man and a woman.
Legally, however, gay marriage doesn't solve all the problems. Ideally, "civil unions", granting all the rights above, would be allowed between any number of mutually-consensual partners - e.g. Mormon polygamy, Hippy communes, ... This would also bring additional recognition and acceptance to these people.
> Since marriage was originally defined by religion
I don't think this is true, and anyway it's completely irrelevant. The post-1950s definition of marriage is a result of secular progress, and was definitely not defined by religion.
> I don't see a problem if religion retains the right of defining marriage - a union between a man and a woman.
I see a huge problem, which is that "religion" isn't an entity. If you want religion to have the right to define marriage, you must allow pro-gay churches to define it the way they see fit, too.
(I should add that as an atheist, I would fully support laws limiting religious weddings to a union between a man and a woman. I'm not interested in dragging religions kicking and screaming into the 21st century.)
Sure, I agree with you on all points. That doesn't mean that marriage equality doesn't solve any issues. It solves one. Solving one problem is exponentially better than solving zero.
As for Mormon polygamy and hippy communes -- absolutely! As long as it is willing, consenting adults, I am all for it! But, right now, lets take this small step.
Marriage defined as a voluntary expression of love and commitment between equals is very, very, very modern.
It wasn't long ago that marriage was also widely accepted as an asset for parents to sell. It was relatively recent that monogamy wasn't assumed, and that the maximum number in such unions was two.
I mean, the original point was that marriage was originally a religious concept (which is, AFAIK, incorrect). Whether the motivation for the union is love or material interests is a different issue, though it is also an interesting point.
Legally, however, gay marriage doesn't solve all the problems. Ideally, "civil unions", granting all the rights above, would be allowed between any number of mutually-consensual partners - e.g. Mormon polygamy, Hippy communes, ... This would also bring additional recognition and acceptance to these people.