Certain things need to be changed for sure.
1) Don't make everyone horny. The flirting between characters should be an on going thing throughout the gameplay and concluded then.
Yes Affinity and character's approval of you is important but it totally feels weird for Gale to be hitting on me randomly that night and being so crushed that I turn him down... its just cringe man.
Have Gale try to flirt with my character, I turn him down, He grumbles. This could be randomly generated event as I play the game. Like the time I stepped in front of the crossbow to protect Sazza. Have him say something that I could respond with a flirt. This should start the series of flirty dialogue lines that both Gale and player experience until the Tiefling party where finally Gale and my character decide to take it further.
That would feel much more organic.
And if it HAS to happen that night then here is option 2!
2) When the party starts Astarion has a question mark over his head about how much he hates the party. And he says he wants to have some fun.
This time instead of telling you wants to bang me how about he asks me "Hey bud, The wine is crap but at least maybe I can find a good bang! What about you? Who do you have your eye on?"
This gives the player total control of who they want. Instead of other characters hitting on you, you tell Astarion "hey I had my eye on Shadowheart, she is hot".
Now whether Shadowheart overhears this coversation or Astarion is the best wingman ever... you take your pick.
Thats when she starts flirting with you and maybe invites you to the "after-party"
Of course this may be a little too quick for some so there is always option 3.
3) Player hits on the tieflings and just has a one night stand. The other Origin companions make commentary and maybe give some flirts of their own.
But besides all that... for the love of god overhaul the Tiefling party, an office party has more energy and fun that whatever it was I saw there at my camp.