Check out this list of free 5e adventures:
List of free DnD campaigns - Dungeon Masters Only - Dungeons & Dragons Discussion - D&D Beyond Forums - D&D Beyond
Just looking at just the WotC-branded adventures: "Death House" has married heterosexual parents. "Elfhunt" has multiple romantic heterosexual relationships. "Hoard of the Dragon Queen" has another heterosexual couple with kids. And that's where I stopped checking. There's no shortage of heterosexual representation.
Thanks for finding those! I guess reflecting on it I think there are two related issues that contribute to the debate. (1) to what extent is the background representation of NPCs/PCs reflect heteronormative assumptions? (2) to what extent are PCs expected to engage in romance as part of gameplay?
Looking at the whole trajectory of the game, (1) and (2) are both high early on...there is the artwork we mentioned, and then the stories that Gary Alan Fine reported (but that isn't romance, is it?) There is a feeling that sex is part of the game and something that is ok for players to talk about.
Then, I think there is a decrease in (2), but less so in (1), vs time. The 3.5 MM, where I started, has a lot of exploitative art. I don't remember to what extent PC romance was a thing then--it wasn't for us. And by the PHB 2014, we have little exploitative artwork but still the heteronormative background material. Given that is the only player facing book and it doesn't have depictions of romance, that doesn't seem to be something the PCs are expected to engage in.
I'll add that a strictly aromantic approach to PCs doesn't negate heteronormativity...as has been mentioned here, not taking a side can be read as defaulting to the norms that are already in place.
The recent push then has been to change both (1) and (2) simultaneously. Less heteronormativity and more PC romance. It makes sense to me that the two would move in parallel especially with the shift in target audience. Given the acceptance of heteronormativity there is less desire to construct fantasy worlds supporting it.