Regarding your Question there are two answers given by the office of His eminence A.sistani:
1- if it was with the consent of the wife herself, that is if she didn't mind, then it is ok but it is Makrooh to perform such an act. (The kirahat is intense)
2- if it was without the consent of the women herself, then it is 'ihtiyate wajib' to refrain from the act in future.
In both the cases there is no Kaffarah.
Reference: mas'ala no. 228
The above can also be referred from the book titlled 'minhaju saliheen'
Moreover, It was highly detested by Imams and there are numerous traditions demotivating this act.
May Allah give us the tawfiq to worship HIM the way he deserves to be worshiped.Ameen.