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When you say necessary be sure to define it and know what determines such a need. If you define it becomes easy to interpret and understand as it varies from person to person. Marriage is high-order societal custom established to form a society based on ethical, moral and legal prospects of accepting a life partner. Through out the times we have seen various reforms, changes in marriage system across the nations, across the religions. Bottom line of all this is to a system of respect for the mankind. Especially to whose who get engaged in such a thing, to put their trust into some one new to their life. Well this is off shooting from answering the question.
Evolution is a continuous phenomenon. Reproduction is part of it. All species in this world, including humans follow this nature's rule. Even microorganisms follow this what is otherwise referred to as asexual reproduction. Although there might be certain exceptions. If this reproduction wasn't there then probably so many generat