Yevamot 56 - What is legally considered an act of cohabitation?
We mentioned that to effect yibum and to acquire his yevama as a wife, one's beginning of cohabitation is sufficient. The same applies to being liable for cohabitation with any woman who is prohibited to him. But what is considered the beginning of cohabitation?
Shmuel says that this is the contact (literally, "kiss") of the genitals. He brings an analogy: imagine a man who places his finger on his mouth; it is impossible not to push the flesh of his lips. In other words, Shmuel agrees that a minimal degree of penetration is required, but he argues that it is inevitably achieved by contact.
However, in the name of Rabbi Yochanan they brought another opinion: insertion of the corona is what constitutes the beginning of cohabitation, with all the legal consequences. What is then considered the completion of the act? - Insertion of the part of the member beyond the corona. What about the emission of seed? - Many say that it is legally not required. What about "emission of seed" mentioned by the Torah? - they say that the situation should be fit to emit it, but not to really do it.
And for what things does he acquire his yevama? - If he is a Kohen, she can already eat Kohen's portion (terumah), which is ordinarily permitted only to Kohen's family, he inherits his brother, he can annul her vows, and he can bury her - which is ordinarily prohibited to a Kohen for anybody but his close relatives.
Art: A Couple Sitting At A Table In An Inn by (after) Jacob Toorenvliet