The nature and relevance of original sin (the idea that all humans are born with sinful natures because they inherit the sins of Adam and Eve) is hotly debated within Christianity. One point of general agreement, however, is that Jesus Christ was born without original sin and did not commits sins on his own account, which is why he was able to offer a perfect life as a sacrifice to redeem mankind.
The most common and widely accepted explanation for Jesus's freedom from original sin is that he was not conceived in sin, as all other humans since Adam and Eve have been. Instead, he was the result of the so-called immaculate conception, the sexless union between God and Mary through the agency of the Holy Spirit. Mary was therefore a virgin when she gave birth to Christ.
Christianity is a broad church, and those on its more liberal wing tend to dislike this explanation of Christ's freedom from original sin, though they have yet to come up with a unified alternative. Liberal Christians argue that sex within marriage, for the procreation of children, is not in any way sinful, but the doctrine of the immaculate conception makes it appear that all sex is tainted with corruption in some way. Liberal Christians also raise (and find it difficult to answer) the question of why God would make sex the means of procreation for all animals, only to circumvent his own laws in a single case.