According to the 2010 Australian Election Study, only about 20% of the population are very strong supporters of the party they say they usually support. So political identity doesn’t seem that important for most people. Yet the same survey included a question on the party identification of the respondent’s spouse, and found that most couples share political allegiances (I included the Greens, but with only 72 married Greens in the sample this is less reliable than the other results).
There was also a question on spouse religion, and for the big religious groups that have many respondents – Catholics and Anglicans – 47% of the former and 56% of the latter have spouses from other religions or no religion.
So even though shared religious beliefs would seem more central to harmonious life as a couple, it seems that shared political beliefs are more common.
I haven’t done more work on the AES sample to explore further, but my first hypothesis would be that despite claims that the traditional sociological bases of the major parties have been breaking down, social background and social circumstances are still the fundamental shapers of political allegiances. And because people tend to marry people who are sociologically like themselves, they end up with shared partisan preferences.