Changing social norms don't change the definition of defamation, but they do change the definition of defamation per se. Just such a change is going on now, as courts consider whether it is necessarily defamatory to say that another person is gay.
The definition of defamation
Social norms do not change the definition of defamation. Broadly spealking, defamation is always a case in which a speaker makes (1) a false statement; (2) about the defendant; (3) to a third party; (4) causing reputational damage to the defendant.
Given that definition, neither of the suggested interpretations is the correct way to look at defamation law (in America, at least). You can say something that is "highly offensive to a reasonable person," without triggering defamation liability, and you can likewise suggest that someone is "deviating from the prevailing norm" without triggering defamation liability.
Instead, the question for defamation liability is basically always going to be whether your false statement caused reputational damage to its subject.
The definition of defamation per se
Normally, a plaintiff must prove that your statement caused him some damage, but in a narrow class of cases -- known as defamation per se -- the court will presume that the statement caused some damage.
Historically, courts have limited per se liability to cases involving allegations of:
- a criminal offense;
- a loathsome disease;
- activities incompatible with the defendant's profession; and
- serious sexual misconduct.
With proof of any of these, a court may be willing to fix liability even in the absence of proof that the defamation caused the plaintiff some monetary loss.
The shifting bases for per se liability
While the categories for per se liability haven't really changed in the last 100 years, societal norms for what falls within those categories has changed.
The category for "serious sexual misconduct" is likely the most prominent example. If the New York Times ran a headline saying "The President is Homosexual" in 1905, it would be received quite differently than if it ran that headline today. Because the stigma was so prevalent, courts treated allegations of homosexuality as allegations of "serious sexual misconduct," and permitted plaintiffs to recover damages even without proving any real monetary loss.
But today, even though there are still plenty of homophobes out there, being gay just isn't as big a deal anymore: anti-sodomy statutes are unconstitutional, same-sex marriage is legal, and so on.
So it shouldn't be surprising that courts have stopped treating homosexuality as sexual misconduct. Most recently, the New York Court of Appeals reached just that decision:
Notably, in New York, the Human Rights Law, since 2002, has expressly prohibited discrimination based on sexual orientation in employment, public accommodations, credit, education and housing. Moreover, marriage between persons of the same sex was permitted in New York years before the United States Supreme Court's decision in Obergefell. The New York State Legislature enacted the Marriage Equality Act in June 2011. Based on the foregoing, we conclude that the false imputation of homosexuality does not constitute defamation per se. ... Furthermore, the additional allegation that the plaintiff viewed gay pornography on the church's computer likewise does not fit within any of the categories of defamation per se.
Laguerre v. Maurice, 2020 WL 7636435, 2020 N.Y. App. LEXIS 8011, 2020 NY Slip Op 07887 (2nd Dept., Dec. 23, 2020).
The rule is not universal -- and to your point, you can probably expect it to be adopted more slowly in more conservative areas -- but courts have begun to accept the proposition that changing social norms have changed the idea that calling someone gay is necessarily defamatory.