I think it's more ridiculous to post about 'religions' and what 'they' think.
There are lots of religions out there. Within individual religions, there are often many points of view, both theological and ideological.
I am C of E. Many Anglicans (including theologians) believe gay people's marriages are valid and should be treated as such, just like straight people's marriages (many others, I am sad to know, do not believe this.)
DH is Orthodox. This is a very, very misogynistic and conservative Church in many ways - yet still, I know devout Orthodox Christians who study and read theology and believe gay marriage is valid and should be treated as such.
I know less about other branches of Christianity and still less about non-Christian religions, I'm afraid. But I would bet they are not all the same.
You need I think to understand someone's theology. In the Catholic Church, marriage is explicitly for the procreation of children. This stance is quite high-profile at the moment (obviously), but even so, there are Catholics who would question the absolute application of that idea, and there are many, many other religions that simply do not agree. In the Orthodox Church, for example, the theological purposes of marriage are to create a family and to provide loving companionship (or a 'little church' within your relationship).
I mention this because it seems many people assume all religions, or at least all Christians, share the same theology of marriage and therefore cannot budge from the same homophobic ideology. This is not true.