I've heard it said that the Levitical requirement for priests to marry virgins is a sign of an assumption that virgins are more pure, which implies that sex is in itself impure. Here is the relevant passage:
And he shall take a wife in her virginity. A widow, or a divorced woman, or a woman who has been defiled, or a prostitute, these he shall not marry. But he shall take as his wife a virgin of his own people, that he may not profane his offspring among his people, for I am the Lord who sanctifies him." (Leviticus 21:13-15, ESV)
There are several things wrong with this argument. One is that the priest is supposed to be pure after marriage too, and if sex is impure then how is he going to remain pure if he has sex with his wife? Another is that there's a reason give, one that doesn't have to do with the purity of the bride but with the offspring. I suppose it's possible to take that as assuming the offspring will be polluted because the mother is polluted, but I don't think that's what's going on here. One of the priestly requirements during Ezekiel's vision of a renewed temple in the last chapters of his prophecy sheds some light on this issue:
They shall not marry a widow or a divorced woman, but only virgins of the offspring of the house of Israel, or a widow who is the widow of a priest. (Ezekiel 44:22, ESV)
If the issue were some animus against people who had had sex, then why would a widow of a priest be ok? Presumably if pollution from sex itself transferred pollution to any offspring, then wouldn't the widow of a priest be just as problematic as the widow of anyone else? This suggests some other reason why priests needed to marry virgins in Leviticus, a reason that must be consistent with marrying widows of priests in Ezekiel. It's unlikely that there's different reasoning involved in the two cases, even if you don't accept divine inspiration behind the two passages.
A much more likely explanation is that the issue with offspring is that virgins raise no problem for offspring having been fathered by someone else prior to the marriage. If a priest marries a virgin, any child she gives birth to will be of the priestly line. If he marries someone who is not a virgin, there is always the possibility that any offspring might have been fathered by someone who is not a priest. At least that's true if her previous sexual activity was with someone who was not a priest. If she was married to a priest, her offspring would still be assumed to be of priestly descent. So this interpretation makes sense of the second allowable condition in Ezekiel, in keeping with the spirit of the Leviticus passage.
Those who begin with the assumption that the Bible is anti-sex like to come up with these implausible claims, and someone who doesn't think carefully about the biblical passages in context can easily come away with the conclusion that these charges have some foundation. Biblical passages certainly do assume a sexual morality that differs from popular views today, but it doesn't follow that the assumptions behind that sexual ethic are anti-sex. Even ignoring the celebration of sex in the Song of Songs and Paul's insistence in I Corinthians 7 that sex should be a normal and regular part of marriage, you still can't easily get the conclusion that sex itself is impure unless you ignore much of the ancient context and often even the literary context of biblical statements.