It's a pointless question... there are very few cultural practices that can't be made to sound at least a little absurd by consideration simply because they weren't a result of strict logic... they are a result of more general trends. They become almost tautological... we hide these things because we value modesty and we value modesty... because we value modesty. For this one, there are a few things that make it make sense.
Biology. For penises and vaginas, there are, for one thing, quite visible examples of arousal, though moreso in the case of a man. Breasts are included largely because they are a secondary sexual characteristic. Plus they can also show signs of arousal. For social creatures, this can easily result in awkwardness or unpleasantness
Hygiene. This is a fairly good reason for the cultural practice. Vaginas bleed on a monthly basis, penises also expel fluids and both can also get infected by outside sources.
Climate. Modesty is often heavily correlated with weather. Areas where near nudity is possible year round generally have looser rules on modest dress and vice versa. This is basically a perception issue... you live in a cold climate, covering up is natural for you, those people in a warm climate don't, they must be immodest. So what starts as a simply pragmatic measure quickly becomes a source of identity
Values concerning sex. Humans are largely monogamous, especially humans in organized societies. Exceptions existed... but they aren't all that common in the grand scheme of things and even polygamous cultures often value fidelity. This makes sense for a number of reasons... in a property based society, it ensures the transfer of wealth to ones own offspring because you know they're yours. Even in other societies, fidelity is valued because clear parentage prevents accidental incest, as well as preventing jealous feuds from developing. Give all that... it makes sense that actions and parts we see as sexual are covered and seen as adult content, because we as a species place higher meaning on sex than simply a means of procreation and one obvious way of enforcing that is reserving sexual exposure to sexual partners
So basically... there are plenty of reasons that can explain why we actively seek to cover sexual organs and characteristics. It's so widespread within disparate human societies that it is clearly more than simply arbitrary distinction.
Oh... and I believe you meant "Raises the question". Begging the question is a logical fallacy with a very different meaning, it requires assuming the conclusion to your argument in the premise.