I'm guessing her scene will come later in the game. For Astarion and Lae'zel, I get the feeling that sex is just the beginning of their romance stories. Shadowheart not jumping into bed with the player straight away just means you'd have to wait a bit longer for your special scene, that's all. It makes sense not to have everyone's romance scenes happen at the same time but occur at different points in the game, depending on the characters involved. Not to mention we don't meet all of the companions right away, either - if none of the existing ones are your cup of tea, maybe one of the others will be.
Waiting often can be good for the writing of something, and for Shadowheart I think the kiss is way more symbolic than a full nights sex. Cause sex for her is for flings, while a kiss may be way more.
Heck, it actually might be cool for there to be a sexless romance in the game. (Obviously not with Shadowheart cause she does mention having lovers...) Like something that is clearly a romance, with all the love and character/relationship development, but for whatever reason there isn't any actual sex. Perhaps because there can't be. I just think it could be an interesting thing from a story perspective, like a romanceable Warforged or something like that.