There is no “Jewish vote.” Jews vote. A small number of them vote for Republicans. Most of them vote for Democrats. Every cycle the Republicans try to improve on their last performance, but Jews are overwhelmingly liberal. So Republicans try to draw them in by talking about Israel, an issue very few of them vote on, but an issue that has the added benefit of helping Republicans shore up their evangelical base.
In the book, Gene Robinson tackles questions like why marriage is an issue for LGBT people, why straight people should support it and why civil unions, while they are a good first step, simply won’t be enough to satisfy the LGBT community.