There are many things that need to be said in the Christian response to homosexuality. This article deals with only one small corner of this debate, but it is an important corner, and hopefully you will remember it and link to it next time you are in some discussion with a supporter of homosexual so-called marriage who, when confronted with the many biblical injunctions against homosexuality in the Old Testament, responds by saying, "Well, the Old Testament says we ought not to eat shellfish and we still do that. You can't pick and choose which passages from the Old Testament you are going to obey." This is the shellfish argument. Let us use two-seconds of common sense to put this stupid argument to rest.
In case you don't know what I am talking about, take this example from Protestant musician Jennifer Knapp, who despite claiming to be a practicing Christian, is also an active lesbian in a same-sex relationship. In an interview about reconciling her homosexual activities with Scripture, Knapp brings up the Old Testament prohibition against shellfish. Knapp says:
"The Bible has literally saved my life. I find myself between a rock and a hard place -- between the conservative evangelical who uses what most people refer to as the 'clobber verses' to refer to this loving [same-sex] relationship as an abomination, while they're eating shellfish and wearing clothes of five different fabrics, and various other Scriptures we could argue about." 
When shellfish is brought up in the context of a discussion of Old Testament condemnations of homosexual behavior, the implication is that Christian opponents of same-sex marriage are hypocrites; the Old Testament prohibits homosexual activity, but it also prohibits eating pork and shellfish - yet we eat pork and shellfish while condemning homosexuality. Therefore, we are being hypocritical. If Old Testament prohibitions against eating shellfish can be discarded by modern Christians, so can Old Testament prohibitions against homosexual activity.
The argument is weak on several fronts. For one thing, it does not tell us what to do about New Testament condemnations of homosexual behavior, or condemnation from the early Church Fathers. It would also be interesting to see how the homosexual advocate responds to critiques of homosexuality by Jews and Muslims, both of whom condemn homosexuality and refuse to eat pork and shellfish. What now, eh?
But the real stupidity of the shellfish argument from a Catholic perspective is in its failure to distinguish between the moral law and the levitical law, which is a interpretive key that is fundamental to understanding the Old Testament.
The Old Testament is full of laws. The rabbis say there are 613 of them, to be precise. Some of these laws are very general and apply to morality and human relations as such, while others are of a ceremonial nature and have to do with the requirements of the Levitical law of Old Testament Judaism.
The moral laws are binding on all men everywhere and at all times; examples of the moral laws are "Thou shalt have no other gods besides me", "honor thy mother and father", and "Do not hate your brother in your heart." The ceremonial or Levitical laws, on the other hand, are binding only on Old Testament Israelites and pertain to the nature of Old Testament worship and discipline. Examples of levitical law are "Do not cook a kid in its mother's milk", "Do not wear clothing woven of two kinds of material", and "Make tassels on the four corners of the cloak you wear". 
Since the moral laws have as their subject human nature as such, they are not abolished with the coming of the Lord and the inauguration of the New Covenant; in fact, they are elevated and perfected. Our Lord demands stricter adherence to the moral law in the New Covenant than in the Old: "You have heard that it was said to them of old: Thou shalt not commit adultery.But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart." 
The ceremonial laws, on the other hand, have as their subject the worshiper of the Old Covenant (i.e., pre-Christian Jews), not human nature as such. These laws necessarily pass away with the ending of the Old Covenant. These laws are transitory by their very nature and reflect the temporary character of the Old Covenant itself. Thus, when the Old Covenant passes away after the coming of Christ, these ceremonial laws no longer have a purpose and are no longer binding on Christians. This was the approach the Apostles took at the Council of Jerusalem (Acts 15) when they judged that Gentiles did not have to observe circumcision or Jewish dietary law.
Another classic example of this is the Old Testament prohibition against eating pork and shellfish. Since this pertained to the ceremonial law, Christians have always been considered free from these prohibitions. This is why St. Paul says "Let no man therefore judge you in meat or in drink, or in respect of a festival day, or of the new moon, or of the sabbaths" (Col. 2:16) and why our Lord teaches "Not that which goeth into the mouth defileth a man: but what cometh out of the mouth, this defileth a man." (Matt. 15:11). This is why Christians have always eaten pork and shellfish with a clean conscience, despite Old Testament prohibitions. It has always been understood that these prohibitions lost their force with the passing away of the Old Covenant.
The crux of the whole issue is whether the Old Testament prohibitions of homosexual actions belong to the moral or the ceremonial law. Because sexuality has to do with the basic nature of man, and because of the moral dimension of all sexual actions, the Old Testament prohibitions of homosexuality have always been included in the moral law. Thus, they are applicable at all times and places and will never pass away. This is why there is really no contradiction or hypocrisy in Christians condemning homosexuality while gleefully eating shellfish and pork. Homosexual acts are always wrong, under every circumstance. This is why the Catholic Catechism says:
"Tradition has always declared that homosexual acts are intrinsically disordered. They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved." 
The Church Fathers understood it this way as well, which is why their writings are replete with unequivocal condemnations of homosexual actions, all while affirming that Christians are not bound by Levitical laws regarding dietary restrictions and other ceremonial obligations.
Note the Catechism reference to Tradition. Some Old Testament scholars have argued that this division of Old Testament laws into moral and levitical is arbitrary, imposed from outside the text, since the Old Testament itself does not give any indication which laws are ceremonial and which laws are moral. This is not entirely true; if we accept New Testament data, there is quite a bit to help us sort the laws out. But by and large, it is Catholic Tradition that offers the definitive clarification on these questions. This is why we cannot throw out tradition and expect to maintain the Christian faith in its integrity.
In short, the prohibition against shellfish belongs to the ceremonial law, which is no longer binding on Christians, whilst prohibitions on homosexual acts belong to the moral law and are always binding. Please remember this article and repost whenever you run across the ignorant shellfish argument.
 Ex. 34:26, Lev. 19:19, Deut. 22:12
 Matt. 5:27-28
 CCC 2357