I think religion/upbringing is a big factor in this.
Along with some people being very "emotional" about sexual intercourse, I can see the fear of not wanting to be known as a whore/slut as well.
I think certain people get very emotionally attached with people they have sex with, I think there is that instinct in us all, but some peoples emotions are stronger, almost obsessive, so I think waiting till marriage is sort of a safe guard against forming strong emotion with someone who will not be with you/marry you. Knowing the person you are sleeping with is in love with you and has taken vows to you only is sort of guaranteeing yourself emotional stability.
There are of course the opposite type of people who are promiscuous, form little emotional bond. Still fine. As long as humans are reproducing!