Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no direct and explicit evidence in Shariah that prevent from sucking a husband’s penis or licking a wife’s vagina. Hence, it is permissible but it is preferable and advisable to avoid such practice since it is contrary to good etiquette and proper decencies. However, it is not allowed to swallow any impurity i.e. Madhi; as for Mani (sperm) one should avoid swallowing it since it is impure according to some scholars, and deemed filthy according to some other scholars.
As for sucking the wife’s breast, it is permissible to enjoy oneself or for the reason mentioned in the question. But it is better to spit out the wife’s milk and avoid swallowing it. However, if some milk reaches one’s stomach it does not harm since the suckling of an adult person does not cause blood relations according the majority of Muslim scholars, including the four Imams and others.
Allah knows best.