I'd dump/divorce them, unless we had kids - in which case I'd probably try to get over it.
20 is a lot in a relatively short period, I wouldn't really want to be with a girl who devalues herself like that - especially when I tend not to get serious with anyone until I know it's going to be a long term relationship. I wouldn't flat out say it's wrong, it's not a crime and nobody was hurt (hopefully!) but it'd clash too much with my own moral code and you can't spend your life with somebody you feel is morally inferior to you.