The Elders did not want Piper and Leo to get married as they felt that it was unacceptable and against the rules. They feared that Leo and Piper's relationship would interfere with their duties for the greater good, the child that they will eventually produce would be too powerful, it would cause even more trouble to follow them.

And Wyatt would grow up to be evil in the future and terrorize the magical and normal world (before Chis intervened). Just about every minute, they had to look over their shoulders because the whole Underworld was looking to turn the boy evil.

Prior to all of that, Patty and Sam, Paige's White Lighter father, hid their pregnancy from The Elders. And before they could get "caught", they gave Paige up for adoption. Paige ultimately lived a pretty normal life despite being separated from the sisters and "forgotten" prior to Prue's death.

But by the same token however, was it hypocritical on The Elders' part considering that Gideon tried to kill Wyatt, they tried to separate Leo from the sisters by erasing his memory, they tried to kill him, the constantly interfered in the sisters' lives, and the refused to revive Prue.