OK, the big question: Would it require recognition of same-sex marriage?
(The following is based on an old soc.history.what-if post of mine.)
There actually is some legislative history on this question. Harvard Law School's Paul Freund (one of those legal scholars who are frequently mentioned as possible Supreme Court nominees--and of course never nominated) testifying against the Amendment before the Senate Judiciary Committee, stated that "if the law must be as undiscriminating concerning sex as it is toward race, it would follow that laws outlawing wedlock between members of the same sex would be as invalid as laws forbidding miscegenation."
http://books.google.com/books?id=Bo0lYVZ5JKUC&pg=PA50 Needless to say, opponents of the ERA like Sam Ervin immediately seized on Freund's testimony and used the specter of "homosexual marriages" as one of their arguments (or, their critics would say, "scare tactics") against the ERA. Supporters of the Amendment, like its chief Senate sponsor Birch Bayh, denied that it would have any such effect. (In general, in using legislative history to interpret a law, a court is supposed to rely on what the proponents of a measure said, not its opponents. Yet to what extent the court should rely on legislative history at all is debatable. Some of the arguments used against reliance on legislative history--e.g., that it is often not clear what the proponents of a bill intended, that some may have intended one thing and some another, etc.--are not applicable here; it is pretty clear that all or virtually all of the congressional supporters of the ERA in the early 1970's did not want it to legalize same-sex marriage. But the fact remains that it is the Amendment itself that the courts would have to interpret, not what its sponsors said about it.)
By 1974 the ERA was losing momentum when it faced a new danger that Freund's and Ervin's warnings might be vindicated. In 1971, John Singer and Paul Barwick, two men in Seattle, applied for a marriage license (as expected, they were turned down) and by the time the case reached the Washington State Court of Appeals in 1974, Washington had passed a state equal rights amendment. Singer and Berwick made a claim that denying same-sex marriage was a form of sex discrimination under that amendment central to their appeal. ERA supporters were relieved when the Washington court decided against Singer and Barwick. See a discussion of his ruling on pages 97-101 of Peggy Pascoe's "Sex, Gender, and Same-Sex Marriage" (in a collection of essays by the University of Minnesota Social Justice group, *Is Academic Feminism Dead? Theory in Practice*)
http://books.google.com/books?id=Y7w7PRzbrAQC&pg=PA97 The *Singer* ruling prolonged the ERA battle a little but did not save the Amendment, nor did it prevent opponents of the Amendment from continuing to use the "homosexual marriage" argument against it. But if it did not have much effect on the future of the ERA, *Singer* did have an effect on the courts: From then on, every challenge to bans on same-sex marriage relying on state ERA's (or state equal protection clauses) lost until 1993.
Only in 1993 did a state court--the Hawaii Supreme Court--decide (in *Baehr v. Lewin*) that a ban on same-sex marriage was indeed sex discrimination implicating its state constitutional provision forbidding denial of equal protection based on sex, and requiring a "compelling" state interest to justify it.
http://www.danpinello.com/Baehr.htm Ultimately, the state legislature in a "compromise" enacted both a comprehensive domestic partnership law *and* a constitutional amendment authorizing the legislature to ban single-sex marriage. Since *Baehr*, the results in state courts have been mixed: some courts have upheld bans on same-sex marriages in spite of state ERA's, others have invalidated the bans (and in some cases the states have rendered the question--as a matter of *state* law--moot by incorporating the bans in their constitutions) but AFAIK they have never been invalidated *solely* because of a state ERA. (I wrote this a few years ago, and will have to check to see if it is still true, but I believe it still is. For example, the Iowa Supreme Court in *Varnum v. Brien* (2009)
http://www.iowacourts.gov/wfData/files/Varnum/07-1499(1).pdf relied on Iowa's state consitution's equal protection clause. More recently of course there have been a flurry of cases declaring laws against SSM unconstitutional under the *federal* Constitution, which of course will presumably be shortly resolved by the US Supreme Court.)
I have to assume that if the ERA had passed, SCOTUS during the era from the 1970's at least into the 2000's would like the *Singer* court have ruled against the idea that it invalidated laws against same-sex marriage. The question of course is whether such a decision would later be overruled, as *Bowers v. Hardwick* was overruled by *Lawrence v. Texas.*
http://en.wikipedia.org/wiki/Lawrence_v._Texas IMO the answer is Yes--but by the time the Court would be willing to overrule it, it would also be willing to at least seriously consider that bans on SSM were violations of the Fifth and Fourteenth Amendments, anyway. So the passage of the ERA *may* accelerate the progress of SSM but probably not by as many years as some proplr think...