The Book of Leviticus is an interesting read, although it should be noted that the shifts in topic can be rather glaring at times. Leviticus 14 seems primarily concerned with leprosy and the response of the community to a leper, and Leviticus 16 concerns the "holy place" (presumably there the Ark of the Covenant was kept), and also discusses sin offerings. And wedged between these two wildly divergent topics is Leviticus 15, which concerns male and female "discharges".

The first half of Leviticus 15 concerns seminal discharge by a man, and concludes its treatise on the attendant ritual uncleanliness that accompanies a discharge of semen with the following observation:

[18] If a man lies with a woman and has an emission of semen, both of them shall bathe themselves in water, and be unclean until the evening.

I don't think it needs to be said that when one is reading the Old Testament, especially the Pentateuch, one should be mindful of the common euphemisms for sex. In the Book of Genesis, the term "know" is the typical euphemism for sexual intercourse; "lies with" is also used quite often, especially in the articulation of the law of Moses.

Possible Objection #1: We could just take "lies with" at its plain meaning, and assume that it does not refer to any actual act of sexual congress.

Response to Objection #1: True enough. But were we to concede that point, we would likewise have to concede that Leviticus 18:22 and 20:13 likewise do not refer to any homosexual sex act. And I think most faithful Christians would disagree with just such a conclusion.

Anti-Marian Christians tend to believe that even if Mary did have sexual intercourse, this would not would not have prevented her from e.g. being called "blessed" of all nations. This is technically true, but where some anti-Marians have gone wrong is with the assumption that, under Jewish law, "sex in marriage is not sinful", that "Mary would have in no way defiled herself by having sexual relations with Joseph, her husband."

Let us come back to Leviticus 15:18, then. The verse would seem to suggest that, according to Jewish law, any sexual act between a man and a woman that results in an emission of semen renders both man and woman unclean until the evening. It's a minor defilement, admittedly, but it is still a ritual defilement.

Jumping ahead to Leviticus 20:10, we note that adulterers — male and female — are to be put to death. Given the absence of mention of any death penalty in Leviticus 15, it is probably safe to assume — since pre-marital and extra-marital sex carry death penalties under Levitical Law (c.f. Levititus 20, almost the entire chapter) — that Leviticus 15:18 refers to normal sexual relations between husband and wife (or between a man and a slave woman, as per Leviticus 19:20-22, but that is another matter).

Let us be clear, then: if a husband and wife have sex, and if there is a discharge of semen as a result of that sexual act, then this results in a minor ritual defilement for both man and woman: both are unclean until the evening (presumably, this means the next evening). So when the concerned anti-Marian Christian objects that proper marital sexual relations do not bring defilement upon the couple, he or she is actually incorrect, and perhaps a bit ignorant of Levitical Law.

I will grant, of course, that Christians do not need to follow all the ordinances of the Law of Moses (although most Christians tend to think that at least a few select ordinances thereof are still binding), because Christ has fulfilled the law. But let us not forget that both Mary and Joseph were Jews, not Christians, and would have lived according to the law. Had they had any sexual relations, they would certainly have observed the tenets of the law pertaining to seminal discharge…and in doing so, they would have been following the extant covenant between God and mankind at the time.

Now, it would be tempting to jump ahead at this point and begin looking at, for example, the Gospel of Luke. However, before we do that, we need to look at the Book of Numbers for a moment, and at what I am sure is, for many Christians, a little-known aspect of Jewish religious devotion: vows of sexual abstinence taken by women, married and unmarried.