Well, I meant Shiro is probably the only one who could have dated someone at the Garrison and it actually be a significant type of relationship (like a fiance). I don't think the others were there for that long. So, the death of someone's lover who worked at the Garrison is used to show how outmatched they were against the Galra. Since who these characters may be attracted too isn't that big of part of the story, anyone else other than Shiro could be gay or bi... like Keith or Lance that some seem to want.
And yeah, the (ex)fiance aspect of it wasn't really shown well enough within the show. You could have just viewed them as dating rather than engaged. Unless I missed something.
I'm not sure if it's the same in this case due to the approach the writers have taken with relationships. Like let's say they said that Keith for sure was straight, but so far that fact wouldn't really matter since romance seems to be one of the last things on his mind and we'd never see anything about it (unless they really want to do something with Acxa). If that's the case, maybe Pidge and/or Coran would be better examples of sexual orientation not being a factor in the story.
I'm not that much invested in romantic stuff with these kinds of stories unless the romance has some kind of negative effect on the overall story... like what was done with Oliver and Felicity on Arrow. So, I'm generally "whatever" about these things. Anyway, the writers greatly underestimated the reaction the death of Adam would get, which I still think is mostly due to the approach they've taken with romance.