I've written about this issue before, so I'm not going to go over my objections in detail. However, it boils down to two issues. First, the same reasons for not having girls in camp outs with boys applies to mixing heterosexual and homosexual boys. Second, I am somewhat confused as to how a boy of the age to be in the Boy Scouts can know that he is homosexual absent being sexually active: at the lower end, the boys are only 11 years old, when they still think of girls as "icky"; and at the upper end of the age brackets, boys are still trying to figure out their identity. And if a boy scout is sexually active, he probably shouldn't be participating in scouting activities; and he is also either a victim or perpetrator of molestation or other sex crime.
On a related note, here is another article (this one being from the Economist) suggesting that acceptance of "gay marriage" must necessarily lead to acceptance of polygamy.
So there's one aspect of the pro-gay-marriage brief that deserves a mental asterisk. A second argument that has always been a bit weak has been the attempt to minimise the extent to which allowing same-sex marriages will change the definition of marriage for straight married couples. When conservatives have argued that gay marriage would "devalue traditional marriage", the response has often been to ridicule the idea that straight people's marriages will change at all. ("OMG! Marriage is now worthless!") This isn't a serious response. Obviously the legalisation of same-sex marriage represents a major change in the institution and in the meaning of the word, much as the meaning of phrases like "all men are created equal" changed significantly when they began to be understood to include, say, women. For people who have a strongly gendered understanding of their own marriage, this is a paradigm shift. The government is now saying it understands marriage as a long-term legal commitment between two people who are assumed to have a sexually attached relationship to each other. Gender is irrelevant; marriage is simply a paired relationship. It's a big deal when social institutions change this way, and if conservative heterosexuals feel their marriages are affected, they're right, even when the way they phrase their complaints is wrong.
Which brings us to moderately off-the-mark argument number three. One of the assumptions that gay marriage calls into question, for many conservatives, is: why pairs, then? If not man-woman, then why not man-woman-woman, and so forth? Again, the response of gay-marriage proponents is generally ridicule. I don't think this is a ridiculous question. "Why can't you marry your dog, then?" is a ridiculous question; marriage, in our society, is between consenting adult persons. (Though states where girls can marry below the age of legal adulthood violate this premise, and show the traces of a premodern understanding of marriage as a reproductive contract between extended families that few Americans would say they support today.) But "why only two?" isn't a ridiculous question. It's easy enough to show that gay marriage does not empirically lead to pressure to legalise polygamy; that hasn't happened anywhere that gay marriage is legal. But this is different from explaining why opening up the boundaries of the 20th-century understanding of marriage shouldn't raise the possibility of legalising polygamy. Why shouldn't it be legal for more than two consenting adults to marry each other?
There are, obviously, a whole lot of societies in the world where polygamy is legal and normal. In fact the anthropological record suggests that the overwhelming majority of human societies have allowed men to have more than one wife simultaneously. I don't want to be taken to be making a creepy dirty-old-man argument in favour of polygamy. But the reflexive belief that polygamous marriages must be evil and oppressive even in societies where they are traditional is basically an expression of cultural prejudice. I would never want to be in a polygamous marriage myself, because I've grown up in the West and it seems freaky and inegalitarian to me; but for people who grew up in Yemen, or in Swaziland, or in Vietnam before the 1950s, that is not necessarily the case. Women in polygamous societies may decide to become a rich man's second wife rather than a poor man's only wife, and do not necessarily feel oppressed by that choice. Their children usually turn out well-adjusted. To take the typical paradigm-upender, if you imagine a Sudanese man with two wives (and children by each of them) who wins the Green Card lottery and is told he has to divorce one of his wives before coming to America, you have to wonder whose interests the government thinks it is defending.