I agree with ask996's reply, as I understand it. To answer the posted question, responses really should focus at least a little on the pragmatic aspects of the problem, not (at the risk of offending, I'll write it!) simply serve up some outdated philosophies and quote passages from religious texts.
As I see it, one big "pro" of gay marriage is that, as ask996 seems to imply, gay marriage would lead to a more equitable extension of some basic rights and privileges (including, but not at all limited to financial and health matters, such as inheritance, partner benefits, and hospital visitation rights).
And, again as I see, one big "con" is that many people would be offended by gay marriage because their long-held views would be challenged. Many people (including several people posting here) seem less offended, for example, if the term "civil union" is used in place of "marriage." That difference in terminology may suggest to them a continued difference in the status or value or meaning of the two types of relationships, straight and gay.
To me, the whole debate surrounding the term "gay marriage" shows the current limits of acceptance among many people. Many of us seem to be stuck in a single track of thought and to be adhering to that deeply flawed and unconstitutional concept of "separate but equal."