It is a known fact that one of the most effective ways in sexually arousing a woman is by playing with her breasts. As such, in general, there is nothing Islamically wrong in fondling with the wife’s breasts. This also includes sucking on the breasts with the mouth.
However, one must remember that it is totally unlawful (haram) and a sin for the husband to intentionally drink milk from his wife’s breast.
The great Hanafi jurist, Imam al-Haskafi (may Allah have mercy on him) states:
“It (milk) is a part of a human being and to make use of it without a real need to do so is Haram.” (Radd al-Muhtar, 3/211)
The above is clear in that the impermissibility of drinking one’s wife’s milk is not because it is impure, rather to take benefit from a part of a human without a need is unlawful. The need here is only for the baby to drink the mother’s milk, as outlined by the Fuqaha.
Therefore, if one’s wife has milk in the breasts and it is feared that by taking them in the mouth, milk will also enter, then it will be impermissible to suck on the breasts. Otherwise, it will be permissible.
Note, that drinking the wife’s milk has no effect on the marriage. Some individuals are under the impression that it invalidates the marriage, which is not true.
And Allah Knows Best