Personally, I dislike "Oh, but I am already with someone else" options and don't use them, even when they are offered, because I think they are cruel. They create a weird sense of competition between the suitors, and devalue the friendship with that person by putting them in a romantic hierarchy. Depending on the suitor, such a reply might also paint a target on your partner's back (I wouldn't put it past Minthara to kill someone to get what or whom she wants, for example.) The one time I really like the "I met someone new" answer though, is when Mystra asks Origin-Gale why he defied her command. Here it's not just about the romance, but about the romance as a meaningful plot device - and it vexes me endlessly that I can't also tell (in my game) Astarion, that he saved my character's life by simply being himself.
But that is something I am missing in general, and why Gale is my favourite travelling companion and romance. With him the relationship is written with some reciprocity in mind. In act 1 he offers you a moment of peace and safety during troubling time, and in act 2 you can return the favour. He compliments your fighting skill, you can acknowledge his magical prowess and intellect, you can exchange compliments, show vulnerability, and all conversations with him draw their dramatic momentum from the flow of arguments. It's just a very rewarding experience, and grants me a feeling of actually doing something together with the character. That's especially true for act 3 when discussing the Crown, which is a conversation I appreciate more, the more I go through it. (Lae'zel is probably a close second when it comes down to how much adore her writing.)
I also wish there were more casual conversation moments. With Astarion-Origin I could chat with Wyll about what it feels like to return home to Baldur's Gate; and during the Tiefling party there was a cute moment when Shart told my character that Gale was sneaking glances my way and we talked a bit about that. Nothing big, but I feel moments like this help to create a sense of camaraderie.
Other than that, I think it's nice that now Wyll and supposedly Halsin (I couldn't really check this yet) only proposition you in Act 2/3 when you have flirted with them during the party in act 1. No need to make poor Wyll sad because you have to reject his dance proposal any longer.
