a Baenre House noble (a would-be matron no less) would lay with a complete stranger straight away?
You seem to forget that when you first meet her, she is worshipping the
Absolute, not Lolth, and certainly not her House. In her eyes, you proved your worth when you led the Goblins and helped her decimate the Grove (as it was the Absolute's will). As for relationship progression, it's not that odd when you consider the harsh reality of their world. If that's not to your liking, that's completely valid, but don't expect Larian to change it. A swift jump to physical intimacy, before slowly making way for romance makes perfect sense to a lot of people.