Global warming does pose a very real threat to many currently living beings. A lot of animals are all but extinct thanks to warmer, colder, drier or wetter habitats. Plus, quite a few new-born humans will probably be alive 100 years from now, if we haven't made the Earth uninhabitable before then.
Gay marriage pose no threat to anyone or anything, whatsoever. It's basically the same as straight marriage, just different sets of genders involved. If you find straight marriages threatening, THEN you're allowed to feel threatened by gay marriages too. Are you also scared of the monster under the bed, dearie?