Am I the only one who doesn't see anything particularly "left-wing" in having sex with children? I mean...of all the institutions that have had problems with that, probably the one that is foremost in people's minds is the Roman Catholic Church, and (recent Pope excepted, to some people) would anyone define the Roman Catholic Church as particularly 'liberal'?
Also, one of the criticisms I most often hear of Christianity from young people has to do with the equation of "traditional marriage" with marriage as presented in the Bible, since so-and-so (Biblical figure) had harems, and so-and-so married a child, and so on. So it would seem that if there's one prohibition that is less-than-likely to change, it's the one against pedophilia, since it's associated even in the minds of the aggressively secular and atheistic with the "stone age" morals of the Bible, which they view as responsible for sanctioning such things and hence immoral. You might see an attempt to humanize those with pedophiliac tendencies (which...I dunno...they're certainly humans; they're just not humans I would trust around children), but as far as an actual change in the law code, I don't see it beyond what is already allowed in certain states (variations in the age of the consent, and so forth). The difference between this and gay marriage is that marriage -- gay or not -- still has minimum age requirements for both parties to be considered to be willingly and freely entering into it, failing some kind of extraordinary circumstance (parental or a judge's approval to get married at a younger age, which is a lot more rare than people freaking out about this probably think it is).