Marriage is a natural state of being* for two people ...Assuming that the essence of marriage is the marriage act (coitus), then marriage is a natural state of being for two people of opposite sex.
Coitus is not the essense of marriage. How do we know this? Simple logic:
Two people can have sex and hold no defining characteristics of marriage: love, friendship, committment, loyalty.
Two people can be married by anyone's definition of the term and not have sex.
Hence, while sex is an important part of many marriages, it is neither exclusive to it, nor essential for it. Therefore it is not its essence.