Fumblings toward a thought or two, in pseudo-axiomatic form.
1.1. The issue of homosexuality within (or without) halachah can be viewed in many ways. One can start with the relevant halachot and work from them "towards" the sociological reality of homosexuality -- that is to say, in a strictly constructionist vein, find those heterim [legal allowances] which might make it possible for the acknowledgment of homosexuality (not homosexual behavior itself) to exist within a halachic community. This assumes that the halachot are applicable, and that moral considerations are either (a) only intra-halachic, i.e. generated by the halachic system, or (b) never applicable to the halachic system even when generated from without.
1.1.1. Halachot are generally applicable. To be a halachically observant Jew means that there is a very strong presumption that the halachah applies in the case at hand. What the halachah is in a given case -- i.e. how that is to be determined -- is very complicated from a general point of view, and I won't go into that here.
1.1.2. There are cases in which the halachah is not applicable. Whether there is a general formulation of all such cases is beyond the scope of this blog. There is an argument among the poskim [halachic decisors] whether every state of affairs can be addressed by halachah, or whether there are some states of affairs which are halachically irrelevant.
1.1.3. Moral considerations are important for determining whether halachah applies in a given case. In opposition to R. David Halivni, for example, I believe that our halachic predecessors (Chazal) did operate with explicit moral criteria. Many of these criteria are explicit (tikkun ha-olam, for one), while others are obscured by literary style. Thus we would not be the first to use moral criteria for evaluating halachah. Whether such "firstness" is relevant is another question.
1.1.4. How moral considerations and halachah interact is not simple. There are some cases in which the moral reality has changed from what was held true according to the halachah as it stands.
1.1.5. Such changes in moral reality do not always require halachic attention.
1.1.6. When large classes of people, by virtue of their wholly moral actions, are regarded as "extra-halachic," or sinners, this requires halachic attention.
1.1.7. Note that 1.1.6. does not apply to egalitarianism, which should be defended on other grounds.
1.1.7. "Halachic attention" entails, among other things, a deliberation whether the halachah as it stands is consonant with the moral reality that we know to be the case.
1.2. Similarly, one could start with the premise that homosexuality is immoral, either because (as noted above) halachic prohibitions establish such immorality by their very existence, or the immorality of homosexuality is a fact of the matter independent of halachah.
2.1. Adult, loving, consensual homosexual relationships are to be morally valued, i.e. are the opposite of immoral.
2.1.1. One can quarrel with 2.1., but this is not a place to rehash the arguments made elsewhere against such spurious claims as the following: homosexuals weaken heterosexual marriage; homosexuals weaken a society's moral fiber; homosexuals are sexually perverse.
2.2. A considerable minority of Jews are gays and lesbians. By 1.1.6., since the halachah as it stands considers them as sinners, liberal poskim (to whom this discussion is confined) should consider the matter closely.
2.3. The halachic ramifications of homosexuality are many, but at the root lies the de-oraita (Torah-level) prohibition of "lying with a man [as one does with a woman]," i.e. anal sexual relations.
2.3.1. Sexual relations are at the heart of a loving, consensual, adult relationship. There are sexual acts proper to heterosexuals and there are those proper to homosexuals. And, of course, there are those which overlap.
2.3.2. The moral dissonance caused by the halachic prohibition of homosexuality does not extend merely to later outgrowths of the Torah's prohibition, i.e. to "recognizing" gay-and-lesbian couples, or the like. Rather, it goes to the very root of the matter, to the prohibition of "lying with a man as one does with a woman."
2.4. A morally based halachic change must take into account the prohibition of anal sex.