Is Phantom’s “boner” when he’s against the portcullis (MOTN) actually canon? What’s supposed to be happening there?
Well, I guess this gets into the question of: what is canonical? What do you consider to be canon? Is it only whatever is written in the text (in this case, the libretto)? Do we take into account the opinions and interpretations of the creators and actors? For that matter, each show has subtle (and sometimes not-so-subtle) differences, so what do we do with that? Are some canonical and some not? Are all simultaneously canonical and not? Are they all separate canons?
I ask those questions because we can look at all those aspects and get some pretty different answers! If we start with the libretto, there's actually nothing stated in there at all for most of 'Music of the Night', which is fairly standard for theatre since it allows room for the creatives to interpret as they wish. Here is all it says:
The Phantom sits at the organ and takes over the accompaniment. [Most of 'Music of the Night' occurs here]... During all this, the Phantom has conditioned Christine to the coldness of his touch and her fingers are brave enough to stray to his mask and caress it, with no hint of removing it. [Mirror Bride reveal takes place, etc.]...
So absolutely nothing there! So wait, where did the "boner" interpretation come from? Why, from choreographer Gillian Lynne, as stated in the Behind the Mask documentary:
He's really, really being powerful with her [Christine] and he's desperate because he feels he's not perhaps getting it where he should and he goes up and he hangs on the grille at the back and she brushes against him and I've always felt that he must have got an erection, of course, and that she probably never felt that before in her life...
Now, the interesting thing here is that Gillian Lynne says that she felt this was what was happening, but she does not precisely state that this was what the actor was directed to do. We do know from the same documentary that she worked pretty closely with Michael Crawford on the show, but did she state this interpretation to him? Did he act upon it? What about other actors; did they receive the same information? So to me, this leaves the "boner" interpretation in a pretty fuzzily gray area in terms of canonicity, in my opinion, because it does seem to be one creative's opinion that may or may not be part of the blocking.
But that doesn't mean that actors haven't heard of it or done things that suggest they are following that line of thought, as we see in this gif with Hugh Panaro and Elizabeth Welch:
Like, the look down from Christine, the way she immediately flees? That's about as heavily as one can imply that she saw a boner without outright saying it.
But then what about actors who don't do that? There could be any number of reasons why those Christines flee. Perhaps they're simply nervous because of their proximity to the Phantom. Perhaps their newly-found sexual feelings are frightening them. For that matter, what about non-replicas, where something very different could be happening?
Ultimately, I think the truest answer is that any number of things could be going on between the Phantom and Christine, and it's up to the interpretation of the actors, the creatives, and the audience themselves. What is canon? You choose! And that's just the nature of theater!