It allows those same-sex couples who would like to be married (not all do, remember) to be able to do so. At the same time, it allows those opposite-sex couples who would actually rather have civil partnerships than marriage to do so.
It doesn't mean churches would have to perform gay marriages, but in the same way as the government (a) defines and governs marriage and its status and (b) performs civil ceremonies for non-religious couples, couples who choose them, or divorcees unable to be remarried in church, government can change the definition in societal, economic, and civil terms and carry them out separately from the religious marriage ceremonies performed by different religions.