That last had indeed occurred to me.
But before S2 even aired, the creators talked sometimes about the ambiguity of Sherlock’s sexuality. They do lie, freely and with relish, but when they are aware enough about a point that early on in a character’s development, it means it’s something they’ve put thought into and therefore something they consider somehow significant.
There’s also their love of canon. They play fast and loose with it, but at the end of the day they stay rooted in it. Canon Holmes didn’t do romance.
Of course, Canon Holmes didn’t do Irene Adler either, but Moffat seems to think he did. And with the references to her that slipped in here and there in S3 (almost all of them showing up in the bits Moffat wrote, how interesting), I think we can rest secure in the idea that Sherlock wouldn’t waste his time with anything less.
(Mind you, I call bullshit on her characterization and, after seeing him have, you know, actual positive and engaging human interaction with Janine, I’ve discovered I actually virulently HATE Irene as the show presents her to us. But it does mean that I highly doubt we’ll ever see him establish a more meaningful relationship with any other woman than he has with her.)
Anyway, even if they did give him an explicitly heterosexual relationship, 1: it wouldn’t be a definitive word on his sexuality (ask me how many gay people I know who’ve dated at least one person of the opposite gender, and that’s not even starting with pan or bisexuality) and 2: I don’t think we would see it take primacy over his relationship with John. Over and over, the show has hammered home that nobody comes before each other for them. Maybe a few rare individuals can stand equal, but not before. Changing that would just detonate the entire point of the show—the one element that forms the continuing thread of the entire story. It’d be like taking House, getting him fired as a doctor, and trying to continue the show with him working at a Barnes & Noble.