Pseudonym J.Z. Eglinton or John Eglinton. Wrote for Mattachine Society (citation needed), later for NAMBLA.
See "Before Stonewall" for some more heinous details.
Believed in his own past lives.
Involved in various religions or cults.
Co-founder of the Society for Creative Anachronism (need good source)
1950s: lives in NYC and was involved with fandom there.
1953 to 1956: Columbia Medical School "superkids" paranormal research, which included immersing kids in NYC sf fandom; working with William Sheldon. Robert Bashlow may also have been involved. (Hard to source this so far; Jack Sarfatti's forum posts, are, well, unreliable).
1954: convicted of child molestation or lewd behavior in Atlantic City; exposing himself to young boys under a boardwalk. Probation.
1962: Seacon scandal in fandom. Anti-Breen sentiment begins to appear in fanzines (sourcing this should be pretty easy)
1964: marries Marion Zimmer Bradley in June. Publishes "Greek Love" with Warren Johansson under pseudonym. Banned from Worldcon (Pacificon II). Arguments in FAPA over his application on the wait list.
1965: founds the International Journal of Greek Love. MZB publishes an article in it.
1968: Breen and MZB move to Staten Island. They help form the Eastern Kingdom of the Society for Creative Anachronism.
Mid-60s: UCB masters in sociology of music.
1979: Keynote speaker, as J.Z. Eglinton at NAMBLA's second converence in New York. (Notes on the talk in NAMBLA News 2 (June 1979).
In 1980, both Breen and Bradley were ordained in the Holy Apostolic-Catholic Church of the East (Chaldean-Syrian) priesthood by Metropolitan-Archbishop Mikhail Itkin.
1985: visit from K----, the 9 year old son of acquaintances from the SCA. His parents check with Elisabeth Waters, Tracy Blackstone, Marion Zimmer Bradley, Diana Paxson & Don Studebaker about Walter; they all agree the visit is fine. source: http://www.sff.net/people/stephen.goldin/mzb/timeline.html
In 1989 Breen was arrested on child molestation charges for molesting K----, after his daughter Moira tells her psychiatrist of the abuse and K---- confirms it to police. He accepted a plea bargain for pleading guilty to felony charge of activity with a boy 14 or younger, with a sentence of 3 years' probation.
In 1991 he was charged with eight felony counts of child molestation involving a thirteen-year-old boy that he had been abusing for four years. He was convicted, and died in prison while serving his thirteen year sentence.