The scriptures and the Church's interpretation of the scriptures on the issue of homosexual relationships are rather clear, I believe.
Relationships yes, mariage - no. There is simply no concept of homosexual marriage in Scripture, like there is no concept of biological evolution, electricity, automobiles, airplanes, protons, neutrons, women having rights independently of fathers or husbands, integrals, differentials, other galaxies...
Homosexuality is a sin according to the Church. Can we just decide that something isn't a sin anymore? People say masturbation can reduce stress and has health benefits (or something like that) so should we then say that it isn't a sin anymore? A lot of people also cheat on their spouses, should we just say no big deal to that since everyone does it? Relationships and marriage are similar, since homosexual relations are condemned in the Bible and by the Church, then how can we allow them to marry?
Or we could recognise that the natural human desires of sexuality have traditionally been perceived as sinful, evil, wrong, etc due to an inherently flawed underpinning philosophy which stemmed from a backwards cultural zeitgeist in ancient Judaea.
Adults who love each other having sex to strengthen their loving bond should not be a sin. It strengthens and expresses love, it hurts no one.
The argument that homosexuals only have sex out of lust is, quite frankly, rubbish. They have sex for the same reasons that heterosexuals do - and you probably don't need to be reminded that even religious married couples have sex for reasons other than procreation. It strengthens their bond and forms an important part of participating in an intimate act of love within their relationship. Sexuality is healthy and forms a positive part of the human condition.
I'm not talking about casual sex, prostitution, meet-and-screw-in-a-hotel relationships, orgies, and other activities which occur purely due to lust. A good case for calling such activity 'sinful' can be made because it involves sex outside of love and committment.
However, sexually active homosexual couples are no different from sexually active heterosexual couples in their motivations for having sex. I'm sure if homosexual couples could have kids, they would; but as we know that's not the reason that the vast majority of couples have sex, is it? I'm sure even the priest at your local parish uses a condom when he has sex with his wife, as does pretty much everyone who doesn't want (or can't afford to support) a new child every nine months. Unless you want to condemn and persecute every heterosexual couple who uses condoms (and thus commit the gross social/moral irresponsibility of Roman Catholics, who tell Africans in AIDS-ravaged nations not to use contraceptives), drop the case.
Monks and nuns take vows of chastisy because intimate human relationships are totally inappropriate for their lifestyle, which is meant to wholly focus on God, Christ and spirituality. Intimate loving relationships are beautiful, but they would certainly distract monks and nuns from their mission, so it makes sense for monks and nuns to be celibate. They are abstaining from a distraction - not something inherently evil. Not all human beings can be expected to live as monks or nuns - we simply weren't intended that way.
The scriptures were written by human beings who were inspired by God, not by God Himself. And like all human beings, the prophets, scribes, Apostles, evangelists, etc were capable of making mistakes or reflecting the cultural attitudes of their time in their works. The deeper spiritual truths of the scriptures don't lie in the literal surface meanings of the words chosen by their authors. The only entity who has access to Ultimate Truth is the Ultimate Reality (i.e., God) himself. Men make mistakes. Unless you actually expect me to believe that God dictated every word of the Pentateuch to Moses on Mt Sinai - an idea which archaeological evidence and textual analysis discredits extensively.
Christianity has the cause of social justice and humanism at its core, not bigotry. Religion has fostered and promoted bigotry for too long, and it has to stop.