Complex sentence: Both are a form of non-monogamy, and the spouses would not view the sexual relations as adultery, although it could still be considered a crime in some legal jurisdictions.
Split into simpler sentences: Both are a form of non-monogamy, and the spouses would not view the sexual relations as adultery. However, irrespective of the stated views of the partners, extra-marital relations could still be considered adultery and a crime in some legal jurisdictions.