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.
She will only have problem if her child would be half-drow (if she ofcourse remains in the Menzoberranzan) that what will summon questions from other nobles.
When was this ever implied or said? Genuinely curious.
Minthara, to me, seems to disregard race entirely after she's freed from the Absolute. She retains some of her beliefs because a huge part of it made her strong, but it's clear she hates drow society and wants nothing more than to be free from it. Also, in her first meeting, she considers herself superior to you
no matter what because she’s the
Commander of the cult you’re stepping into. This point of view still holds true during that night of celebration.
Anyway, I assure you all, one night of mildly confusing sex after just being freed from mind control isn’t the worst that could happen in the world of Faerun. She took that small moment of clarity to spend the night with you, someone she holds to a high regard because of your aid. If you think that's weird, that’s entirely your opinion, and you are free to refuse her offer.