LGBT marriage, at least in the United States, is a legal definition that need not have any bearing on how 'marriage' is understood in any other context. It came about purely as a legal hack, to address the needs of people who sought some legal acknowledgment of their stable companionship. The existing law and custom provides this wrt. "married" couples, and this was simply extended wholesale by sort-of pretending or establishing by fiat that two people can be 'married' no matter what their gender, as far as the law is concerned. It's a sensible mechanism that has plenty of other uses in law and policy.