Yevamos, Chapter 6, Mishnah 1. This mishnah deals with the act of yibum itself, and teaches that the intention at the time of the act is not what determines the matter: however the act is performed, the yibum takes effect.
The text of the mishnah:
"Haba li-yevimto" - one who comes to his yevamah for the sake of yibum, in any of the following ways:
"Bein b'shogeg" - whether unintentionally, thinking she was a different woman.
"Bein b'meizid" - whether intentionally, having no intention at all of fulfilling the mitzvah of yibum, but rather acting out of mere licentiousness and a chance encounter.
"Bein b'ones" - whether under compulsion, that he was forced to come to her.
"Bein b'ratzon" - whether willingly, having intended properly for the sake of the mitzvah of yibum.
In every one of these situations the mitzvah of yibum is fulfilled.
The combinations of intention between the man and the woman:
"Hu shogeg v'hi meizidah" - he is mistaken as to her identity, while her own intention is licentiousness and not for any particular purpose.
"Hu meizid v'hi shogeges" - his intention is licentiousness, and she thinks he is a different man.
"Hu ones v'hi lo ones" - he is compelled and she is not compelled.
"Hi ones v'hu lo ones" - she is compelled and he is not compelled.
And apart from matters of intention, the same applies to the act itself: "Echad ha-me'areh v'echad ha-gomer" - whether the one who performs only the initial stage, where only part of the organ is inserted, or the one who completes a full act. In all of these ways - "kanah" - he has acquired her.
The meaning of this acquisition:
By virtue of this act he has acquired his yevamah and made her his wife. Some explain that this does not refer only to acquiring the woman, but also to acquiring the inheritance of her deceased husband, which takes effect at the moment she becomes his wife. And since she is his wife, if he wishes to divorce her, he can do so only with a get.
The mishnah concludes: "Lo chalak bein biah l'viah" - there is no difference in the type of relations, whether in the natural manner or not in the natural manner; either way the mitzvah of yibum is fulfilled through her.
In summary: In this mishnah we learned that the act of yibum takes effect regardless of intention and circumstance - unintentionally or intentionally, under compulsion or willingly, and in any combination of the two; and regardless of the manner of the act as well - whether the initial stage or the completed act, whether in the natural manner or not in the natural manner. In all of these he has acquired his yevamah as his wife and acquired his brother's inheritance, and he can send her away only with a get.