Kleptogamy (klepto- ‘stealing’ + gamos ‘marriage’): dating or getting affectionally/relationally involved with a friend’s’ partner(s) or (an)other committed folk(s).

It’s also defined in biology as “a behavioral pattern in which males from outside a group, or lacking in status within the group or attractive characteristics, attempt to mate with females from the group, often by means of some deceptive/cheating/sneaking behavior”.

This posts description bases the concept of talarica(gem), but not essentially coviting. Note this can be a consensual relationship orientation as a form of nonmonoamory. – Ap