Regarding the other issue, the ALP was shut out of government throughout the 1950s and 1960s because it's most conservative faction was thrown out of the party and decided to hit back in a fantastically spiteful manner. That sort of history is quite the incentive for not trying that type of thing again.
Yes, of course, but the question is rather
why are there so unusually many anti-gay people in the ALP in the first place? Is there something about Australian culture that differs from much of the west when it comes to this?
In most western countries - and pretty much everywhere that is most comparable to Australia - Social Democratic and centre-left parties are strongly in favour of gay marriage.