Yes, at least in the US, cyber sex, seducing a child online, online grooming of a minor, viewing or sending child pornography, etc. is all illegal. It is no different than being in person with the minor. Online or in person the minor is a minor and even if they believe they know what they are doing the law says they cannot give consent.
If caught the adult would be considered a sexual predator just the same as if they were caught offline. I hate to admit it but I know three men who have been caught and charged.