I'm trying now to remember the fellatio scene in Das Rheingold.

Is Fafner involved?

You're both wrong. Don't you remember Alberich's otherwise mystifying lines

"Feuchtes Nass
füllt mir die Nase"

But Alberich's scenes feature a dwarf, not a goblin.......

My grandmother's maiden name was Swallow. And yes, I'm afraid I am talking about my paternal grandmother.

One of the four things I remember about seeing "Powder her Face" (on tour in Ghenth) is what must be the end of Act I: um... how can I word this gently... 'the Duke pleasuring the maid from behind while exciting himself by looking at a photograph of his wife performing fellatio on another man.'  Yes, that's about it...   

I wasn't to sure what to think about it, it was the first season I ever went to the opera.  On the way to the foyer I overheard a young man saying to his girlfriend 'that this wasn't really a subject matter for opera'...   
The funniest thing was the little decrepit old bus standing outside the theatre after the performance, collecting the cast, and seeing the singer who performed the Duchess sitting quietly and well-behaved in her seat, with her purse on her lap. 

No fellatio, but there was a masturbation scene in the opera I mentioned briefly on the Belgiana-thread: "Frühlings Erwachen" ('Spring's Awakening') by Benoît Mernier, an opera about the tribulations of youth.  Lucking (or 'Unfortunately', one might argue) the masturbation was staged symbolically: just a young man sitting in a cupboard with a lot of blankets, singing erotic fantasies, while his imagined lover tilts the closet slowly to the ground...  Mernier cites music from Cavalli's "Eliogabalo" during the 'masturbation', an opera rediscovered lately and first performed in 2004 in Brussels, in a production by Vincent Boussard, who directed and helped to conceive "Frühlings Erwachen" as well.  A slow, chanty piece for string quartet only and one of the most magical moments in the entire opera.

I believe too that "Frühlings Erwachen" is the first opera ever to have a homosexual love duet, unless someone can correct me (and trouser roles do not count!).

IIRC it's a masturbation competition in the original Wedekind play, isn't it? (Well, I'm fairly sure I do RC, at least in the production I saw. It's not the sort of thing you readily forget ).  A heartbreaking play.  

Staying with Tippett, the 'Achilles' scenes in KING PRIAM contain something very close to a gay love scene too.