1. It's against our instincts to "mate" with a close relative because it lowers genetic diversity and heightens the chance of birth defects. It's pretty much hardwired, like liking big breasts and wide, child-birthing hips. I mean, sure, there's deviation and taste differences, but for the majority it's pretty much a fact. 2. Sex can be very emotional, and sex with a close friend can be very complicated. Sex with a relative can be even more complicated and emotionally distressing. Could you honestly -blam!- your mom or dad and still look at them as a parent, caregiver etc.?