I romanced Astarion twice now (basic Tav and Durge) and had none of these buggy problems with Gale. BUT the key is to nip Gale's romance in the bud SUPER EARLY, still in act 1.
There was a scene at the tiefling/gobbo party that got moved a rest or two after the party, the conversation where he talks about Tara: to nip that romance completely, you have to choose the very last option once he's done reminiscing:
"If this is an attempt at flirting, you should stop. I am not interested."
There is an option where you state that he is a treasured friend (and I guess people pick that one the most) and I believe that is the one that gets him super horny, for after you pick it he talks about having the pleasure to enjoy your company in the morning... Like legit, both times I completely avoided Gale's romance with the option I stated above.
