In 2000 the Baptist Union produced a study booklet 'Making Moral Choices' to help Baptist congregations explore a variety of moral issues, including sexuality. There is much in there that is helpful, and it remains a valuable resource for congregations. Sadly it is no longer available through the Baptist Union online store, so a pdf is available for download below.
The Baptist Union 'Declaration of Principle' is the basis of the Baptist Union of Great Britain. It is relevant to the topic of same sex marriage because it sets out the terms on which churches covenant together to form the Union.
In 1996, four Baptist College principals wrote a study on the Declaration of Principle. Twenty years after its publication, it still speaks powerfully about what it means for Baptists to belong together, even when they disagree.