Marriage brings together two individuals to become one. You do not solely own yourself anymore. Your body becomes your wife's. Your wife's body becomes your own.
I have often wonder, why is sexual relationship pleasurable! Is it because God wants to encourage us all to indulge in sexual intercourse? Probably, it is NO!
If it is a YES, it would not have been a sin to fornicate, nor would it have been a sin to commit adultery. Right?
The sexual union was planned by God in the beginning, to be between married couples. That has not changed.