Drow don’t view sex in the same way as other races. She wants Sarevok as her bitch, not a partner in a meaningful relationship. Viconia doesn’t have a problem if you sleep with whores in the back rooms of the Copper Coronet and she naturally expects you to view sex the same way she does. If you say the right things to her through the romance you can actually shift her alignment to good (or not, if you prefer her evil
).
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Nature’s first green is gold,
Her hardest hue to hold.
Her early leaf’s a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.