how come relationships/weddings..

True that. But a guy who is a doctor could be the biggest scumbag out there. What makes you think a guy who goes to clubs, drinks, sleeps with women wouldn`t continue these activities after marriage.
While a guy who never went to a club, drinks, is a virgin, mostly likely has good intentions esp if you are aboard b.c of easy access to these things. There are guys out there that want their first and last love to be their wives. You know how hard it is for a man NOT to do these things while all your friends out drinking, clubbing with their girlfriends and you are home b.c of your morals.
As a far a treating crap could happen. I mean you are a religious guy and you end marrying a girl who wants to be all modern and dress bad (shows cleavage, legs) , goes to clubs, wants to hang out till late could damage marriage and i have seen those happen to ppl.