There is an option to bed both characters actually, if Gale is flirted with during the "Weave scene" and/or "Loss scene" but the player sleeps with Astarion (or another companion but I can only confirm this scenario) after, then, the following night (I believe), Gale offers to sleep with him as well.
This is an intended scenario/option given that there are lines of dialogue where Gale acknowledges what happened (the flirting, the sleeping with someone else).
And this, indeed, seems to glitch out the bedtime story.
Ah! Well I learn something new here every day. I don’t think I’ve ever seen that, but then I don’t tend to spam rest to get those scenes of Gale’s unless I’m intending to romance him at the party. (I have had him express interest at the party but not got this option, but perhaps that’s because I’d not flirted with him, or perhaps it was because I slept with someone other than Astarion when it happened.)
Apologies for my misleading reply, OP!
