The '60s are a little ways out of my normal bailiwick, and the references to it in the standard literature are few. "The Boondoggle and Beyond" in Joe Sanders' (ed) Science Fiction Fandom (pp.85-86) basically repeats the facts from the Fancyclopedia, with a little less focus on Breen's homosexuality and a little more emphasis on the impact on fandom. There is a definite whiff of the "Lavender Scare" of the whole thing - and while fandom was perhaps a little more open to homosexuals than the American public as a whole, homophobia was still apparent at least into the late 1940s, as several derogatory statements toward homosexual sci-fi fans are apparent in Francis T. Laney's Ah, Sweet Idiocy! (1948).

At the same time, there may well have been genuine concerns that Breen would molest or attempt to seduce the younger male science fiction fans at the convention. Breen wrote and published the book Greek Love (under the pseudonym J. Z. Eglington) in 1964 which focused on the historical pederasty of ancient Greece, differentiating sex between an adult male and a minor from that of two adult men; under a pseudonym he contributed to the Journal of Greek Love articles on the same topic. He had been previously convicted of child molestation in 1954, and received a suspended sentence. His wife Marion Zimmer Bradley - who was herself eventually accused of child molestation and statuary rape - said in a 1998 depostion about the 34-year-old "Breendoggle":

Q. You knew that Walter was a pedophile at that time?

A. I knew it. Intellectually, yes, I knew it.

Q. And you knew that he was sharing a bed with a 13-year-old boy at that point; correct?

A. Yes. But, as I say, I believed Walter was impotent and nothing that could bother anyone could happen.

Keep in mind, most of the fans involved with this affair didn't know the facts; I haven't been able to dig up much from the fanzines that clarifies what the convention organizers knew about Breen's activities, but scuttlebutt in letters was typically very hard and fast in fandom, the moreso in Los Angeles and other cities with a highly-organized fandom presence.

The accusation that a prominent fan was a child molester and banned from a convention would have been a bolt from the blue for most fans, though. This was only a decade or so after the McCarthy hearings. People chose sides, the sides entrenched, fandom became split.

Given that this was over 10 years before the foundation of NAMbLA, how significant of a following did those ideas have?

Public attitudes toward sex between adults and underage partners (of either sex) were somewhat different than they are today. Openly, it was illegal and there was a lot of social outrage--when it came out that the 22-year-old Jerry Lee Lewis had married his 13-year-old cousin in 1958, there was a huge backlash, just to take one prominent example. But child marriage and sexual relationships that would be considered sexual molestation and statutory rape today were more common and sometimes legal within living memory in the early 1960s.

Pornography during the era, especially erotic novels, were much more open about focusing on underage subjects going through or just past puberty as potential sex objects. The 1943 film Child Bride, which is nominally an educational film and yet was often screened as a sexploitation film because of the use of a nude young girl, emphasizes both the cultural shift away from child marriage and the ongoing attraction that taboo to the American public, or at least a segment of it.

The "Greek love" portion is more difficult to answer. Certainly, Breen was not alone in using the example of ancient Greece (both real and imagined) as an excuse for his behavior, and possibly as a model for his relationships. How widespread the concept was is more difficult to say; it was not, as far as I am aware, stated in NAMBLA terms, although I believe some of the essential talking points were influential on later NAMBLA literature. Breen has a write-up in Before Stonewall: Activists for Gay and Lesbian Rights in Historical Context if you want to read a little more on that.