I think ingame marriage is really a interesting topic.
I played some games with marriage system.it seems most Asia MMORPG have a ingame marriage system.eg.lineage2,Flyff,and some Chinese games. west games are much different.WOW,guildwars are have no marriage.Imo,good-made marriage system add lots of funny.
marriage system has various implement.some game it just give you exp bonus when grind with your couple.some give you marriage skills like teleport,heart defense(buff) ,sacrifice(take attack for couple),outrage state(become rage and powerful when your couple dying)  love combo...
some game marriage have a advance children system.you can have a child ingame and cultivate he/she as you wish.a grown up child can help its parents in various way...and that's not all.the most important thing ingame marriage give is just a feel i think.it makes you feel not alone,more relative to the game...
But marriage brings some problem yet.
Guy marriage are not allowed in most ingame marriage system.this make sense in Asia games.since guy marriage still not widely acceptant in Asia.but its much different in America.any imgame marriage must deal with this problem.
Another problem is age.many young kids play games.their parents surely don't wish them to involve in that things.