Assuming the problems with that change don't become obvious within the time period. For an example of this happening, look at the problems caused by say no-fault divorce.
To stick with your example I don't think I've seen mainstream conservatives notice anything of the kind. Do keep in mind how I use conservative in this context and how I differentiated them from traditionalists worthy of the name. Now besides the traditionalists and conservatives you have other currents of right wing thought who notice such things, but they are pretty marginalized. A few blogs on the internet focusing and analysing this problem is unfortunately a very minor phenomena unlikely to result in social change.
A thought occurred to me today as I skimmed an article in a rationality forum where the subject of gay marriage cropped up; seeing as the issue has been hotly contested in various public fora and especially the courts, what about poly? After all, many if not all the arguments for gay marriage apply to poly marriage as well.
Questions for LWers who are currently in a such a relationship, or have an opinion to share:
Do polies want to marry each other or do such relationships not lend themselves to permanence above a threshold of partners? Should polies campaign for the right for a civil union anyway? what are the up and down sides of this? etc