Sex (or reproduction if you prefer) is natural to keep the human race going. It's states in the bible to go and populate the world. It's only a sin when lust and adultery come into play.
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And just pointing out (cause if I don't, someone will) there is no harm in enjoying sex inside of marriage. It helps keep a healthy relationship, sex isn't only used for reproduction ya know? The bible also states that intercourse was given to us to enjoy with our "wives", and "husbands".