Basically this. Whether or not it's LGBT or straight isn't the point. It's how phoned in it is.
Bleach for example -- Ichigo ended up marrying Orihime with no real build up to that ever happening on his end. Yeah, she loved him romantically, but we barely saw anything similar from him. But apparently him blushing around her is enough to say he loved her.
It's stupid.
Personally, you ought to change this topic to "Stupid Last Minute Romances".
Because that's essentially what this topic is, and stories with both LGBT and/or straight MCs suffer from this.
It ain't just LGBT -- It just feels like that because more stories with LGBT characters have been more frequent now and suffer from this bad writing. But this has been happening for a while.
If romance wasn't clued in from the bulk of the story, and there's no build up, don't try to include it now.