In addition to the inherent structures of a human society geared to heterosexual mating and reproduction, the social pathologies of the gay community pose another obstacle to the liberal dream of a society where sexual orientation makes no difference.
Right-thinking people tell us that these pathologies are all or mostly due to oppression by straights and, thus, that they will evaporate as soon as that oppression ceases, but--taking a lesson from the experience with blacks--this seems unlikely.
Certain social pathologies arise directly from the unbalanced nature of homosexual sexuality. Male and female human sexuality evolved together in a complementary fashion. This complementarity is lacking in homosexual relationships. Yes, there are variations between individuals which mean that any given couple, homosexual or heterosexual, can be more or less complementary in various ways, but the differences between male and female sexuality are quite significant when groups are taken as a whole and individual variations cancel each other out. This means that, taken as a whole, different patterns will be seen in male-female, male-male, and female-female sexuality.
The most obvious manifestation of this is the hyper-promiscuity of a significant portion of the gay male population. We can get a glimpse of what sexuality tailored purely to male tastes looks like from straight male porn: endless random couplings that get right to the point and leave no attachments. Of course, this is a fantasy world, based on the absurd premise that females have the same libido as males, but gay men actually inhabit (or can choose to inhabit) a sexual reality that at least approximates this. Being gay does not make you a sex maniac, but male sexuality unbalanced by female sexuality tends toward sex mania, with the attendant venereal diseases. A significant number of gay men have an astronomical number of sexual partners, and the HIV infection rate remains high despite the fact that everyone knows what causes it and how to prevent it.
Not being a woman, I don't know how this unbalance is likely to work out among lesbians, but there are bound to be problems there as well. Lesbians are obviously more likely to form lasting relationships than gay men, but I recall a study showing that lesbian relationships were still significantly less stable than heterosexual relationships. So, it does not seem to be the case that more monogamous inclinations of human females relative to males leads hyper-monogamy in female-female relationships, but that there is something out-of-balance in female sexuality as well.
Perhaps opening marriage to homosexuals will, over time, change homosexual culture to be oriented toward monogamy like heterosexual culture. Gay boys will grow up dreaming of meeting Mr. Right and settling down. There's probably some truth in this. Some gay folks (mostly lesbians), will desire the settled life of marriage (and even children), and this will become a goal for some from early on, and the gay sexual market will be modified accordingly.
However, the disappearance of marriage among the lower classes makes me think that monogamy is a fragile thing that arises and flourishes only under certain circumstances. It is not "natural" in the sense that, when obstacles are removed, it just emerges as the default. Rather, monogamy is an arrangement that arose specifically around the yin and yang of male and female and is specifically oriented toward rearing offspring. Everything else (including--to some degree--old straight people getting married), is an imitation of the "real thing"--something of a tragedy or a joke. Other kinds of couples can be cute, endearing, edifying in their devotion to each other, etc., but they are not engaged in the work of family. Yes, gays can adopt or lesbians can be inseminated like farm animals, but this presupposes the weak modern ideal of family as a mere launching pad for individuals bound for the corporate/state machine. There is no essential connection of blood that ties all the individuals together and little reason for it to remain permanent.
