I disagree with it, but conservatives, with their belief that, "how things were is how things should be", are consistent in believing this (since that's pretty much the definition of a conservative).
Let's be clear here. In terms of family structure, very few conservatives believe that:
A) That what we'd call an ideal structure is something that matches every circumstance. It is after all, an "ideal".
B) That we used to universally have such an ideal structure.
C) That a "nuclear" family somehow excludes the extended family situations that have existed for hundreds of years.
The problem is that politics tends to push for simple slogans and answers, especially in a democratic society. So you have conservative politicians talking about "family values" and liberals talking about "human rights", both of them equally nebulous and misleading concepts.
As a conservative myself, I have nothing at all against adaptation and progress. The only thing that defines my conservatism is to proceed cautiously and to not step over lines just because they are there to step over. That certainly does not mean heading back to any sort of Dark Ages or turning back the clock. It does mean that we don't pull our arm out of joint trying to grasp things that we can only barely reach. It means considering the ethics behind decisions and creating a workable framework to make decisions *before* jumping in and unleashing something new on society. It also means saying "No" to certain things that could have long term dangers even if the potential for short term satisfaction or gain is high.
Of course, there's more than one type of conservative out there, just like there is more than one type of liberal out there. There are some fruitcakes that fall under the conservative umbrella and some that fall under the liberal one. However, there are few conservatives that actually do fall under the extremist label and no two are perfectly alike. It's unfortunate that all people of one general grouping or another get tarred with the same brush.