This doesn't surprise me. I think the split between the parties should be about the same as it is in the heterosexual population. I'd expect it to be somewhat skewed because the Democrats give the impression that they will do some things to advance gay rights.
But that shouldn't have too great an effect because: 1. Gay people are interested in all the issues, not just who purports to care about specific gay-related issues, 2. There isn't much reason to trust the Democrats to do more than pretend to care, and 3. If the GOP seems libertarian, it may appeal to those who mainly want the government to leave them alone to live their lives according to their own values.
The 31% figure is probably also skewed by who is willing to tell pollsters that they are gay. It may be that more conservative respondents are less likely to convey the information and, especially in the case of religious conservatives, less likely to admit to themselves that they have a homosexual orientation.