I mean, you are "naked" and doing kinda porn content, in a studio with at least !0 people, cameras and stuff, the nerve and maybe the pressure can get you off guard and the blood pumps and boom! You have a boner, because happen to all men in similar context of pressure.How many people think he got a semi hard doing this scenes?????
Im not saying "is because Dylan is bisexual" he can be whatever he wanna be, is not the matter here, but for being straight and nailed a gay sex scene so perfectly, you must be in the mood of the context: "Im now in the flesh of a gay dude, fucking another dude". And he is a great actor because of that.
Maybe Im wrong, I don't know, and maybe when the movie releases and we get properly quality image and audio, maybe we can see or maybe not (probably no because never happens) But we can have hope.