The truth is, religion is responsible for a lot of unreasonable behaviors, especially in matters of sexuality and pregnancy. For a few hundred years, the Church considered a natural act of sexual reproduction evil and strongly linked exposed skin with sexuality. This is all, of course, completely idiotic when you think about it. Why exactly being nude is not acceptable, what should people cover certain parts of their bodies when it's not cold? And why shouldn't media depict people doing so? It turns out, it's been caused by Christianity and it's association that "any nudity=temptation". In their eyes, when thinking about sex, you're not thinking about God. So it's the Church's fault that sex was taboo for so long, they essentially tried to make people stop thinking about it, or (since eliminating sex is impossible) consider it solely as an act of reproduction (which is not really a good idea). If it wasn't for the Church, people wouldn't object about somebody being nude when it's approbate (and being depicted doing so in media), and we'd be talking about sex much more openly.
And of course, that's not mentioning their ridiculous stances on things like abortion or in vitro, in other words scientific (heck, sometimes even medical) interference in human reproduction.
Blame the Greeks and Romans
Christianity's extreme prudishness about sex is a reaction to the levels of debauchery of the old Empires.
This might be useful for explaining why exactly Christianity has the stances it does on sexuality. Most stupid ideas started out, in fact, quite valid, but that was 2000 years ago. Church is unable to adapt to the modern world, so I hope that it'll someday be laughed at as ridiculous by most people. Since ancient times, influence of gods and supernatural beings steadily decreased, which can clearly be seen in literature. The sooner this process arrives at it's logical conclusion, the better.