Wiktionary
extramaritally
adv. Outside marriage.
Usage examples of "extramaritally".
There's also a male scientist's better-genes-through-cuckoldry theory, which reasons that a cavewoman with the misfortune to have been married off by her clan to an ineffectual husband could use her constant receptivity to attract (and be extramaritally impregnated by) a neighboring caveman with superior genes.