I can think of a few, actually. They all happen to also be gender-flippers, though. Some I thought were handled better than others.
Choice of Robots: There’s always an opposite-gender romance available; Tammy or Silas. (There are some slight differences between the two, including completely different appearances, but they’re mostly the same character.) In this case, there’s a note in the code that they’re always opposite in order to tempt the most people into going down that route. So I really don’t think this succeeds at its goal… wouldn’t it have made more sense to match, as first priority any romance option that had previously been picked, and as second priority, whichever character you’d chosen near the very beginning? 
Those would be much clearer indicators of preference.
A Midsummer Night’s Choice: The Maroon Knight is always the opposite gender to you. I might’ve enjoyed a same-gender version there, but I can see how it fit given the plot and setting.
Tally Ho: If you have a binary gender, Frankincense is always the opposite; if you’re nonbinary, Frankincense is the opposite of Rory. So I’m not sure if Frankincense is strictly heterosexual, exactly. Anyway, this is an implementation I thought went particularly well, especially being balanced with Rory, who’s always same-gender if you’re binary, and chosen if nb. Very balanced.
There are also characters who are bi but have a preference, like in Zombie Exodus: Safe Haven, Woody is easier to romance if you’re female, but can be romanced by anyone. This can be a way to increase variety in orientation without shutting anyone out.
Anyway, I’m not against exclusive orientations as long as it’s balanced. If it’s written so that someone playing as gay is going to have a worse experience than someone playing as heterosexual… maybe don’t do that?
As a shameless plug, @ParrotWatcher’s The One Chosen is currently slated to have two nonbinary ROs, plus one character who matches the MC, meaning three nb romances if the main character is nonbinary too! Though there’s going to be a bit of a wait in the story before romance actually starts for anyone.
Though if you have mostly bi/pan, with a few single-gender orientations, that would cut down the number significantly. And I have seen some games with very high numbers of ROs—though those often aren’t particularly locked anyway. (E.g. Zombie Exodus: Safe Haven, which I think only has one fully locked character so far, plus some with preferences. Or Keeper of the Sun and Moon, which doesn’t have any gender locking.) It does take a fair amount of writing, but when the author’s really into it, it can make for a pretty lively cast.
Okay, well, I don’t want to dogpile too much here, and I can see from later posts that you’re not promoting this attitude, but… that is a really harmful attitude. It’s exactly the sort of thing that can make being gay rather unnerving—and I can tell you, the risk of “someone might lash out at me” is rather worse than the risk of “I might be asked out.” All it means is the gay guy (or bi guy, for that matter) is interested in going out. It doesn’t mean he’s assuming you’re gay… it just means he’s not assuming that you must be straight. I generally don’t make assumptions about what people’s sexual orientations are. If I do, I’m bound to be wrong. Also, there is nothing wrong with being gay. It is not an insult.
This standard would require that everyone verify others’ orientations before ever asking anyone out. I don’t see heterosexual guys doing that each and every time.
Of course, there are ways of hitting on people that are bad and/or threatening no matter who’s doing it. If someone’s not taking no for an answer, or behaving inappropriately, then yes… that’s wrong, and it’s still wrong even if a gay guy is doing it to another gay guy, or any other configuration… being a straight guy hit on by a gay guy has nothing to do with it.
Also at one point, I did, possibly, sort of, ask out a guy who turned out to be heterosexual. It was a bit nebulous whether it was just a friendly hangout or what. Anyway, he just sort of subtly worked into the conversation that he was heterosexual, and we kind of just left it at that. It’s not too hard to handle such situations gracefully.
As far as games go, having a character be interested or ask you out—provided this character doesn’t know your orientation—is fairly realistic. Something like a forced kiss, on the other hand, isn’t really the best approach for any configuration.