I can imagine a scenario in which a child in a warm, functional family sees his or her parents making the sign of the beast with two backs and thinks nothing more than "Aww, ain't they cute? They must really love each other!" with no trace of embarrassment or anxiety for the viewer or the viewees.
Then again, I can also imagine a scenario in which vampiric elephants from outer space conquer Earth.
There are a huge number of reasons why kids can get royally freaked out seeing the old folks doing the horizontal rhumba. Some, I agree, are the product of unreasonable cultural conditioning, but other revulsion I believe is perfectly natural.1
Picture the following situations:2
You're four years old, and you find your parents' bedroom door closed. You hear muffled sounds within. Curious, you reach up and turn the doorknob. The door swings open to reveal your father, naked and sweating, on top of your mother. Your mother is moaning as if in pain, and your father is pushing against her violently. He's hurting her! It sounds like he's trying to kill her! You start to wail in fear. Your father turns, and barks, "Close the door, dammit!"
You're twelve years old, and painfully body conscious. You push open the door and see your parents making out on the couch. They're mostly naked, and their flesh is all old and saggy and wrinkly. Your father looks like a beached walrus under your mother. A dozen dismayed thoughts run through your mind: Will I look like that when I'm that old? No, I'll never look like that. They look ridiculous. Gawd, why don't they cover up? You quietly leave, blushing furiously.
You're fourteen years old; you're at an age where a good stiff breeze gives you a woody. You get a thrill out of looking up "dirty" words in the dictionary at school. You stumble in on your parents making love in the den. Your mother's breasts are bared; she doesn't look like the chicks in Penthouse, but she looks ... good. You feel yourself getting hard before you realize that holy shit, I'm turned on at the sight of my parents having sex and you hurry out of there fast as you can, wishing you could wash your brain clean of the memory.
You're sixteen years old. Ever since you hit puberty, your father's been acting creepier and creepier. He keeps "accidentally" barging in on you while you're bathing or changing clothes. Sometimes, he comes into your room at night and ... you don't want to think about it. It was just a dream. But as you see your father groping your mother in the laundry room, it all comes flooding back, all those bad memories you've tried so hard to put out of your head. You run away, wishing you could purge your mind.
If our culture was more accepting of "mature" body types and if sex wasn't an act most kids see only in movies, an act where the onscreen lovers are never anything but beautiful, then, yes, in some families embarrassment would lessen.
And if children were not so sheltered from normal sexual activity, they'd be less likely to find the sight of their parents having sex to be inadvertently titillating (and therefore horrifying). However, I feel that we psychologically have some built-in revulsion to seeing our immediate family members -- whether parents, siblings, or children -- as sexual creatures; such revulsion prevents unhealthy inbreeding, after all.
But if a kid has experienced molestation or unwanted advances at the hands of a parent, there's nothing that's going to make them feel anything but incredibly ill at the sight of said parent having sex. For that matter, any child who has been molested, whether by a family member or a stranger, runs a high chance of having a bad reaction to anything that could remind them of the abuse, be it an object or the sight of naked adult bodies. And since our culture already condemns child abuse, and it still happens more often than any of us would like to believe, it is unlikely to be purged from our society.
Thus, no matter how utopian our societal views become, for some kids seeing their parent(s) having sex is going to make them want to scrub their brains. For the vast majority, of course, seeing the 'rents aardvarking isn't going to turn them into a pillar of salt. Temporary embarrassment is usually the worst that happens.
1 I am not implying that "natural" is synonymous with "good" or "desirable" here; I'm talking about behaviors that seem to stem from any innate human emotional reaction, be those emotions constructive or destructive, rather from a reaction to cultural pressures. The distinction is sticky, I know, and I'm not a psychologist or sociologist, so your milage may vary.
For instance, it's human nature to feel frustration -- which often turns to anger -- when one is interrupted during sex (not everyone feels this way, of course; some people don't care for sex at all and might be glad to be interrupted). It's human nature to become embarrassed, and to want privacy to some degree (name me a culture in which people commonly have sex in public; I haven't heard of one, thus I feel we have a universal need for some amount of privacy. The old era of having sex in the family bed was, for them, the best they could do for privacy). Scientists have found evolutionary bases for evil human acts such as rape and murder, so those are arguably "natural" as well.
Is overcoming instinctive bad behavior through being taught to behave in a more enlightened manner more "natural" than just giving in to baser instincts? I can't speak to the relative "naturalness" of the behaviors I've described, and can only say that I think the scenarios are pretty common.
2 All these scenarios are based on firsthand accounts I've heard from other people. They will not apply to every child or teenager (see footnote 1), but that should go without saying.