First, "consider" does not mean to put in practice. Second, in the case of underdeveploped and weak countries there two main reasons to follow an example of the big brothers.
1) "Cargo cults". Sometimes some countries want to become "civilized" but having no smart leaders they try to imitate "civilization" by adopted some of trendy moral thingies. For example, Japan (which is historically much less obsessed over controlling sexual urges of its citizens then Anglo-Saxon countries) introduced anti-pornografic laws to imitate Anglo-Saxons. Blessfully as Japanese are quite smart people their dropped this absurdity after several decades and now Japan is one the best examples how erotical production should be dealt with.
2) Easy way to win some sympathy and maybe some benefits. After all, why not allow some harmless sillyness if our international partners are so obsessed with? If Vietnam introduce same-sex marriage it will magickally become more democratic without really sacrificing anything important.
Well, should I then dismiss any idea I do not like on the premises that it had certainly already occured to somebody else?
Everything is separate and worthy debate with different issues. But the point is that the "debate" of same-sex marriages is not the most important, and if its proponents want to promote it in countries which are skeptical of LGBT specialness and chosenonnes they should join their forces with others who could be interested in amending of "marriage" or introducing more flexible "civil unions". Taken alone the cause of homosexuals wanting marry much less important then issue with cousine/sibling or polygamous/polyandrous marriages.
One is not prohibited by the law to make consentual sex with any relative (cousins, siblings etc) and do not risk jail for doing this. Same-sex sex is also allowed.
First cousin marriages are actually legal in most of the world. US is shameful exception in the civilized world. They should have dealt with it first before advancing to same sex marriages.
I'm not against cousin marriage but why is it so much more important than same-sex marriage? It seems like a big part of your objection to same-sex marriage is that gay people are some insignificant minority that no one should care about, but I think there are a lot more gay people than people who marry their cousins or want to.
And gay rights is about giving gay people the same rights as everyone else, not promoting "specialness and chosenonnes" I think the last one isn't even a word.