As a society we have defined certain boundaries, one being the age of sexual consent. People are not allowed to have sex with minors because as a society, we find this to be repulsive, and have made that action illegal.
There is nothing illegal about "swinging". It does not fit in with my beliefs, but I do not impose my beliefs on others. As a matter of fact, I defend the rights of others to believe differently than I do.
All in all, regardless of the parent's acrions, the criminal here is the person who took the little girl, the parent's sexual preferences did not give him (or her) license to abduct a child.
It is not illegal to own Pit Bulls. If a child in my neighborhood is the victim of an attack by unknown dogs and I own Pit Bulls, I will expect to be questioned by authorities.
Most child abductions that are not because of custody disputes are sexually motivated. That makes any peculiar sexual behavior by the parents quite germane to the investigation.