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Thanks ! But My Question is about GAY/ LESBIANS; Not Heterogeneous Sexual Relationship.

What Will happen to Human Race if it is Legislated That Men Marry Men and Women Mary Women as it is already Happening In Some Part of the World??

That is covered by the first part of my answer.
More marriages will be issued, wedding bakeries and planning companies will make a bunch of money.

What else would happen?
Same sex coulpes already exist and live together in the world, whether you make it legal or not.