Do you think it is all right to have sex before marriage? - Yes
Or do you think it's in the hands of the individual? - Absolutely.
Do you think it should be based on the morals and principles of the couple/individual? - What? Umm, having sex without thinking about that is kinda like ..rape isn't it? Can't force that Jack!
Should they respect their parent's wishes? - It would be nice if people listened, but it is up to the individual whether or not they choose to abide by that.
I think it is a person's choice and right. Follow your path and what not, do what you want.