The reason is obvious.
Parents are ashamed of sex because they're ashamed of their bodies.
When Dad sees a shirtless, ripped Brad Pitt on a screen, it undermines his masculinity, and when Mom sees a buxom starlet in full body make-up, grinding on some dude, it undermines her femininity.
The Father and Mother see the child as a token of their sexuality, which vindicates their sexuality.
Subconsciously, they don't want the token of their egocentric false sense of sexual power to be bruised by the sight of sexual superiors.
Also, when children come into play, Mothers are ashamed because the bond they feel with their children is so strong that it almost crosses over into sexual attraction.
This is because,
1) All people are narcissistic and they usually only have sex with people to whom they're sexually attracted, so when they procreate, the result is a little genetic "perfect blend" of themselves, and their sexually attractive mate.
2) the intense intimacy of carrying that subconsciously sexually attractive being inside your body, and then having it suck on your swelling breasts (which gives you a sense of importance (all physical stimulation aside) drives mothers insane.
So, because parents honestly love their children, their subconscious sexual attraction to their children must be completely mentally shut out.
Since they consciously know that they are sexual beings, the only option is to attempt to de-sexualize their offspring.
No normal parent is afraid that their child might enjoy cutting off Jango Fett's head, but they're terrified that their child might become sexually aroused.