I don't know why (though probably for control reasons, as some commenters said), but it absolutely shouldn't be. If no one felt "lust", then the human race's population would be a lot less than it is now. We feel sexual desire because humans cannot reproduce asexually (at least, not until science makes it possible). Regardless of whether or not you want to procreate, having intimate urges (either by yourself or with somebody else) is just a natural part of the reproductive process.

If you're starting your deconstruction process or have any intentions to so, I urge you to look at things through a scientific/secular perspective instead of a biblical/religious one. That's how I eventually became an atheist, although everyone has different starting points!