Kamasutra 1.5.20

1.5.20

mamā:mitro vāsyāḥ patyā sahaikī:bhāvam upagatas tam anayā rasena yojayiṣyāmīty evam:ādibhiḥ kāraṇaiḥ para-striyam api prakurvīta.

(m) My enemy is a friend of this woman’s husband, I shall therefore cause her join him, and will thus create an enmity between her husband and him.

For these and similar other reasons the wives of other men may be resorted to, but it must be distinctly understood that is only allowed for special reasons, and not for mere carnal desire.