Not a liability: National Republican primary, very conservative states
A liability: National general election, very liberal states (VT, MA, etc)
We are really nearing the end of opposition to LGBT rights in the US. The public already supports ENDA (I think even Romney supports this?) and the repeal of DADT by huge margins and we're seeing the shift towards supporting gay marriage which will lead to the end of the Defense of Marriage Act (and civil unions).
The speed on which things happen will vary. The fastest I think is if Obama moves on from supporting civil unions to supporting gay marriage (again), wins reelection, and Democrats hold the Senate allowing him to appoint more Supreme Court justices and California holds off on legalizing gay marriage at the ballot in favor of letting Perry v. Schwarzenegger reach the Supreme Court who would hopefully give us a Loving v. Virginia-like ruling legalizing gay marriage nationwide. It could be over this decade.
I'll be kind of sad once it's all over. I don't think there will ever be another type of issue that enrages the religious right so much 
Don't be glum, chum. As biotechnology and Computer Science becomes more and more sophisticted, we will be give all kinds of fun, new ways to piss off fundies.
