Christian churches should absolutely allow gays to be ordained as priests. But "should" is the key word here. They certainly shouldn't be forced to do so. Affirmative action has never applied to careers in the Church, to my knowledge. I am starting out on this debate with the idea that any kind of sex, for priests, is prohibited anyway.
Although the Church makes no claim to base its decisions on logic, we must ask ourselves why one's sexual orientation would make any difference at all? As the Church has evolved, restrictions placed on priests inevitably included prohibitions on sexual activity. This ensured the priest did not have a family and thus be less available to the Church, and it also was meant to cause the priest to prioritize spiritual experiences and devotion over sexual experiences and family/spouse devotion.
Priests, in the eyes of the Church, should not be engaging in sexual behavior at all. Therefore, the Church can't be justified in denying an otherwise great priest a position in the Church based on previous sexual behavior. There is no rule stating priests must be virgins; the vows of chastity are in effect once the priest is ordained.
How, therefore, can the Church logically discriminate against gays who would like to be priests? If gay priests are to be held to the same chastity vows and rules as straight priests, sexual orientation should not matter; there shouldn't be any sex going on anyway.
Consider this: A straight man joins the priesthood. He has had relationships with women in the past, before he was ordained. His sexual orientation means nothing now. In becoming a priest, he has given up sex. Now, consider a gay man wants to join the priesthood. He is just as educated and dedicated to the Church as any other candidate. He has had relationships with men in the past. His sexual orientation also means nothing once he is ordained. He, too, has vowed to give up sex and focus on religion.
An example of a religious organization which allows gays and lesbians to be ministers is the Unitarian Universalist Church. This, however, is a Protestant sect, in which ministers are allowed to have families. In fact, when a minister has a family, he or she, in Protestantism, is usually seen to be a better candidate for minister than someone who does not. The reason for this is that the ministers often act as counselors and provide guidance to their congregations. If your minister knows what issues affect families first hand, or has had experience with marriage problems, he or she would make a more ideal counselor, and bring such knowledge to his or her sermons.
The Catholic Church, however, tries to uphold standards that are simply too high. A priest who is basically charged with developing the mentality of a eunich cannot fully understand family and marriage problems. Neither can he, for that matter, understand the troubles everyday people face in the work environment. Because of practices like banning sexual relations therefore, banning priests from having children and families, priests become people who are increasingly distant from their parishioners. They hand down advice, but their advice is not always readily accepted by the vast majority of people, because it doesn't seem applicable to daily life.
If the Church wants to gain more followers and keep itself giong strong, it should consider allowing priests to have families. This is not to say priests should be encouraged to frequent bars and strip clubs. It means that if a priest is in love, he should be able to have a family. The term "family" applies to both gay and straight relationships. The Church likes to stress to people the importance of family values in society, but the decision-makers in the Church do not, themselves, have families. What better way to honor and encourage families than to allow the priests themselves to have their own?