why does s-e-x is important in the relationship??

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i am always wondering why do its always important to everybody?? why they want it?? for giving thier physical needs?? and why does they are always saying, if one relationship got no sexually interaction to one another, they end up in separation,?? is it really true?? coz i dont really think that s-e-x is really important to our relationship.... can you give me an explanation bout this?