Because it's a roleplaying game and you can play a murder hobo or an ass, if you like. I don't mind that there are options for being a horrible person, I do mind if I am forced to be a horrible person (like with the old brothel dialogue for Gale, which just offered no good way out). For characters who are sincere but think that feelings are oh so hard to put into words, it is nice there is a second chance for a declaration of love at a later stage though.
In the greater context, there is also the idea that your character might feel the need to go along with the romance because it keeps Gale from having suicidal thoughts - the morning after dialogue acknowledges this option too. Many of these ideas are probably not the most romantic and I agree with you that the whole night (not just the kiss) is very important to Gale, but - for me at least - one of the great strengths of Gale's quest that it works really well as a narrative which allows the player to portray a wide range of characters and positions towards Gale and his situation. So being given the option to be open and honest or take a none verbal approach seems like good story telling. I think it's not a separate scene either because Gale himself needed his declaration to be special and he doesn't even think that creating a magical meadow for you is special enough - it is much more than just a romantic night, it tells you about his flaws as well as his hopes and needs.
It is noticeable though that while you can be extremely cruel or at least very rude to all the male companions and even to Lae'zel, you can hardly even be moderately impolite towards Shart or Karlach. Not that I wish to, but if you compare what you can fling into the direction of the guys and Bae'zel to the dialogue for the two girls, there is a stark difference. When Karlach wakes you up in the middle of the night to tell you that she has the hots for you, there isn't even an option to be someone who is grumpy because they were just woken up for something that could have waited til morning. Which I would consider a very viable reaction in this situation.
This double standard is, what puzzles me and makes me wonder why. Unfortunately I only have very cynical answers to this question and I'd rather not go down that route.
