JDWActor: I'm sorry to inform you but your argument is flawed, not just in the absence of marriage in-and-of-itself, as this is obviously a prerequisite for sexual intercourse as dictated by the Catholic Church, but also in the fact that a married couple is only fulfilling one of the two purposes of the conjugal act.
Firsly, since homosexual intimate relationships are not considered valid relationships by the Roman Catholic Church and hence inelligble for marriage, the entire argument of validating homosexual relations is defunct.
Secondly, when considering the Billing's method, one must understand that the ONLY way to ensure that a pregnancy does not occur is ultimately by abstinence. The Billing's Method does advertise 99% efficacy but that 1% leaves LIFE in God's hands. One of the main purposes of marriage, and the Church would say THE main purpose is procreation; to bring up and teach children in the ways of Christ. One must remember that any sexual relations still allows for the possibility of pregnancy, however slight those chances may be. A pregnancy can still occur even during a woman's menstrual period, though the cases are very rare.
A homosexual relationship can never allow for the possibility of new life, and hence it does not fulfill the primary requirement of sexual intercourse. Although love is an important factor; it is secondary (albeit a very close second). The unitive principle of marriage expressed by the union of sexual love can of course continue after a woman no longer produces ovums or in the case of sterility of either person but that does not mean the possibility of "un-natural" unions under this natural order. A child is a gift, not a right, as indicated in the Catechism 2374 and in the stories of Rachel and Jacob and of Joachim and Anne. Thus to argue that infertile couples who love each other would equate to homesexual couples who love each other is erroneous.
Further, infertile couples are encouraged to pursue prayer for a child and permitted to accept "research aimed at reducing human sterility,..., on condition that it is placed at the service of the human person,...,according to the design and will of God." Catholic Catechism 2375. These latter methods are not available naturally to homesexual couples, unless third party donors are admitted, and this is against the teachings and designs of God.
Finally, sex is something that can be abused even in a marriage. Lust is not negated in a marriage, and sexual intercourse is not an absolute goodin a marrage. If couples are continually practicing the Billing's Method, never trying to have children, then they are in direct violation of God's will, the intentions of marriage and the promise they made to each other on their wedding day.

This is not meant as a sermon

but meant to enlighten one on the Roman Catholic stance on homosexual intimacy and that of heterosexual married couples. One can't split hairs with theology, a sin is a sin no matter how you look at it. It is the severity of the act that dictates the punishment. So, in the case of homosexuals acting out their sexual desires, this is considered a mortal sin, in the eyes of the Church.

novalis: Un-natural is not my word but the word of the RCC. My node is entirely the view of the Church, not once did I mention what I felt, believed in or agree with; my opinion is frankly quite irrelavant. I was just trying to shed some fact on the matter, what is listed in the Code of Canon Law and the Catechims of the Catholic Church. The references speak for themselves.