in my humble opinion *OPINION being the key word here*....i think its adultery if its non-consentual sex outside a committed relationship of any type..be it marriage or otherwise. my Master/Husband and i have, what i like to call a "contolled semi-open marriage" in where, if one of us wishes to indulge in sex outside of our own relationship..it is discussed first, and agreed upon by both...not agreed on? doesn't happen. simple as that.
just an opinion