He behaved adulterously.    From Wordnet, Princeton University. 

Adjective : an adulterous relationship.    From Dictionary.com. 

If she miscarries, the child was conceived adulterously.    From Wordnik.com.  [reddit.com: what's new online!] Reference

They'll run adulterously after other lovers in Exodus 32 in giving their hearts to a golden calf.    From Wordnik.com.  [the blue fish project (dave bish)] Reference

"all monstrous, all prodigious things," cuckoo-like, adulterously lay their eggs, and brood over, and hatch them in the nest of every neighbouring State.    From Wordnik.com.  [Selections from _Letter to Noble Lord_] Reference