I have a theory on this. First, it's not just Christianity that's against sex. It's arguably one of it's most vocal opponents however, so, yes, it does deserve some special mention.
My theory however is that we, human beings are rational. Non-human animals aren't. They may exhibit signs of rationality, but, by and large the view of many previous cultures and belief systems is that what chiefly seperates us from the lower species is our ability to raise ourselves out of the state of nature - the Hobbesian hell, as it were. Guess what, sex is inherently animalistic, isn't it? There's no rationality in sex or in attracting women - which is why we often have so much trouble understanding it all.
So, where is this rant headed? Well, since many of these earlier cultures (look to the Victorians for instance) were so concerned with keeping themselves out of that dreaded state of nature, they eschewed anything remotely animalistic - especially sex. Sex is primal. Sex is barbaric. Sex makes us no better than the lowest of species right? Therefore it's something to be ashamed of.
What a terrible terrible attitude. Unfortunately because of the way we've developed as a species we're caught in a pickle. This aversion to sex has been so engrained in our psyche's that even now when we all know better than to say that sex is wrong or bad, it's still tough to rise above it.
Christianity (as a Catholic myself I know what of I speak) is an ancient organization, isn't it. Dates back 2000 years or so. So we're talking about an institution that has remained more or less unchanged for two millenia! Two thousand years ago we were obsessively trying to distance ourselves from the animals. That's just a reflection of that. We're special in God's eyes. We not only strive to be more, if we aren't more we're failing this creator. Therefore not only does sex make us animals - it makes us immoral. And it puts us all at risk in regressing to Hobbes life that's "Solitary Poor, Nasty, Brutish and Short". Of course we want to avoid that!
All hogwash of course. But I think it at least attempts to explain where we are, and why we're here.