As this table reveals, perhaps the most dramatic change to occur is that a large majority of Americans now believe that gay and lesbian sexual relations are morally acceptable. Public opinion on this issue has shifted in a major way since 2001. Back then, most Americans believed that same-sex behavior was immoral; however, that has changed alongside the important social and political gains made by the LGBT+ community in the last few years, including the fact that same-sex marriage is now legal throughout the nation.

Sex between unmarried men and women and divorce were rated as the most morally acceptable behaviors in 2001, and they continue to lead today. By contrast, both consensual non-monogamy (specifically, polygamy) and non-consensual non-monogamy (cheating) continue to be the least accepted.

That said, it is interesting to point out that the number of Americans who think polygamy is morally acceptable has more than doubled over the last 18 years. It would be interesting to know whether moral acceptance of other forms of consensual non-monogamy—by which I mean polyamory, open relationships, and swinging—has changed over this same time period. Unfortunately, Gallup has not regularly inquired about the acceptability of these other relationship configurations, which means we can't really say what, if any, changes have occurred. I would hypothesize that their perceived acceptability has increased as well, but we need more data to know for sure.

It’s also worth noting that while attitudes toward abortion have remained remarkably stable since 2001, attitudes toward stem-cell research have become quite a bit more favorable during that same time period.

As for the new issues asked about in 2013, we can see that there have been some slight shifts in the moral acceptability of porn use and sex between teenagers; however, most Americans appear to find these issues morally unacceptable (which is interesting considering how common these behaviors are—most people have sex as teenagers and most people use porn at some point). By contrast, use of birth control is almost universally perceived as morally acceptable, with more than 9 in 10 Americans saying so.

With all of that said, it’s important to highlight that while Americans' sexual attitudes overall are increasingly liberal, the numbers in the table above reveal that many Americans continue to hold traditional and conservative sexual values. This cultural divide is, of course, also regional, with attitudes being more liberal in the northeast and west, and more conservative in the south—and that helps to explain why sexual morality issues are so much more controversial in some states than others.