I liked laila zubairi's point of view. You should make your kids understand the difference between right and wrong.
and if you don't like what they choose its fine to tell them but don't impose your liking on them.
and if their decision turns out to be wrong then they know they only have to blame themselves.
and Orye maqbool jan saheb, parents should be there for their kids even if they want to marry by their choice, you recognise that women are vulnerable physically then why abandon them for marrying somebody, consequently they will not be abused.