It's like gay people voting Republican.
I actually understand them, and likewise ethnic minorities and poor people who vote GOP. They usually share a LOT of the GOP's espoused fiscally conservative ideals. They just happen to differ on particular social issues.
IMHO, if you're a single-issue voter, you're probably voting for candidates who will do you more harm than good. But if that issue matters deeply to you, voting against it is extremely difficult. In a sense, it is like a philosophical "pearl of great price".