From the Canonical Defender comes this:
Possible canonical consequences for participation in ‘same-sex weddings’
March 19, 2012
British Catholic blogger Damien Thompson recently remarked “For Roman Catholics the prospect [of Britain legalizing same-sex marriage] is a very bleak one. Even if a liberal priest wanted to do the honours, he’d incur automatic excommunication and be out of a job. The ‘wedding’ would be a parody of the sacrament. So a legal ban would save a lot of awkwardness.” Of course, there is no such thing as ‘same-sex marriage’ and civil law should not pretend that there is. But that’s not what I respond to here. My focus is on canon law.
As canon law reads right now, officiating at a ‘same-sex wedding’ does not result in latae sententiae excommunication. Green, “Table 1”, in CLSA Comm (1985) at 932. To the extent, however, that such action on a cleric’s part might constitute, say, “abuse of ecclesiastical power or function”—and I think that such an act would constitute abuse of Church office—he might well face punishment “according to the gravity of the act” and even loss of office. Canon 1389. Whether that punishment could, in turn, in the face of, say, clerical recalcitrance or repeat offenses, lay the foundations for later excommunication (Canons 1393, 1399, and/or by particular legislation under Canon 1315) remains to be seen.
Individual Catholics attempting such marriages seem generally susceptible to a “just penalty” for simulation of a sacrament under Canon 1379.