While married to another player and spend time together you lvl up a new set of bonus rewards and some new titles?
marriage and also divorce titles
Heartbreaker - divorce 10x
Casanova - marry 20 times
Romance exp bar and extra bonus lvled by the amount of time spend together
Lvl 5 (480 hours together) - magic attack +5, physical attack +5
lvl 10 - increased exp,spirit gain +2%
lvl 15 - Physical defence +5%