Does JESUS not clearly define what the definition of marriage is in these verses?
(Matthew 19:3-9) And Pharisees came up to him, intent on tempting him and saying: “Is it lawful for a man to divorce his wife on every sort of ground?” 4 In reply he said: “Did YOU not read that he who created them from [the] beginning made them male and female 5 and said, ‘For this reason a man will leave his father and his mother and will stick to his wife, and the two will be one flesh’? 6 So that they are no longer two, but one flesh. Therefore, what God has yoked together let no man put apart.” 7 They said to him: “Why, then, did Moses prescribe giving a certificate of dismissal and divorcing her?” 8 He said to them: “Moses, out of regard for YOUR hardheartedness, made the concession to YOU of divorcing YOUR wives, but such has not been the case from [the] beginning. 9 I say to YOU that whoever divorces his wife, except on the ground of fornication, and marries another commits adultery.”