Regarding having sex with a woman during her menstrual period, the topic is not addressed directly in the New Testament, and so the conclusion one draws about the matter depends largely on the method one uses to interpret the Old Testament.
The Law of Moses
There is not universal agreement about how to treat the Law of Moses. Some groups claim that it must be followed (except where impossible or instances where it is superceded—like offering animal sacrifices, because Jesus was/is our atoning sacrifice). Others attempt to distinguish between laws that don't apply and laws that do. The third major camp says that the old law is not binding on Christians, that it was for Jews, i.e. Christians aren't compelled to obey anything in the law.1
A seemingly unstated premise of your question is that Christians read passages from the Law of Moses (like the ones to which you've referred) as being commandments they ought to follow (but don't).
The prohibition of homosexual behavior doesn't come exclusively from the Law of Moses (e.g. Lev 15, 18). It's stated explicitly there, but that is not necessarily the best argument for the idea. English translations of the Bible have New Testament authors prohibiting homosexual behavior (e.g. 1 Co 6:9, 1 Tim 1:10). This is much less ambiguous regarding the source and applicability of the commandment. There is some debate about shades of meaning of the Greek words involved, but it's a more logical place to begin the assertion that homosexual behavior is bad.
Principles for Deciding
In general, the principle by which Christians decide (among those who decide, and not those who automatically accept the conclusion of ordained clergy) goes something like this:
- Is the matter (or a related principle) addressed in the New Testament?
- Is the matter (or a related principle) addressed in the Old Testament?
- Is there some historical (Christian) precedent that addresses the matter?
- Is there a logical answer that does not conflict with the New or Old Testaments?
Some groups place less/no emphasis on history/tradition, but nearly all groups place emphasis on the New Testament over the Old (where there might be two mutually exclusive rules—animal sacrifices are one example), and Scripture over history/tradition. Generally, Scripture is the authoritative source, followed by historical precedent. Finally, logical reasoning is fine in the absence of (or to make specific use of) related principles in the Scripture or applicable historical precedent or tradition.
1 This doesn't mean the rule is bad or that it's OK to do what the law prohibits. It's just that because it's in the Law of Moses is not a compelling reason to follow it—just like people in Armenia don't have to obey the laws of Mexico because those laws apply to people in Mexico. Murder is against the law in Mexico, but that's not why Armenians aren't allowed to murder. Murder is illegal in Armenia—and that's why the Armenians aren't permitted to do so.
