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Typical passengers have a required attraction threshold somewhere between 700 and 1200, but characters with sex as one of their hobbies only require half as much (350-600). To find passengers with sex as a hobby, look for ones who are touching themselves (the two specific descriptions to look for are are “… is gently stroking her crotch” or “… is tracing a finger around her nipple.”).
Also, characters who are hoping you’ll ask them for sex will be blushing or unable to take their eyes off of you.