This really comes down to the idea of Other. Anything outside of the commonly accepted (e.g. Judeo-Christian) construction of "coupling" (i.e. mating and long-term relationships) is deemed wrong. Furthermore, what is classified as Other are relational (i.e. homosexuality is akin to incest, bestiality, pedophilia, necrophilia in that it is "not normal or healthy").
This, of course, is a fallacy. Especially when you consider that most incestuous relationships are probably heterosexual. Wouldn't that make incestuous relationships more akin to heterosexual marriages?
Seriously.
But on a more technical note: incestuous relationships would have problems in some cases of procreation. Cousins have fewer problems than other more closely related incestuous couples. Though I suppose certain couples could adopt or get sperm donors or surrogate mothers from outside, as do homosexual couples.
Socially, homosexual marriages should not be compared to incest where the incest in question is criminal or morally questionable (as inter-generational, etc. was mentioned above). Homosexuality is a natural occurrence that should not be forced into social abjection merely because others feel it's unnatural, dangerous, damaging, or a threat to the status quo. That's ridiculous.
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Knowing that death is certain and that the time of death is uncertain, what's the most important thing?
—Bhikkhuni Pema Chödrön
Humankind cannot bear very much reality.
—From "Burnt Norton," Four Quartets (1936), T. S. Eliot
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