Mishnah 5 in Maseches Yevamos comes to reinforce what we learned in the previous Mishnah regarding the saris (a man incapable of fathering children): the saris "lo choletz v'lo meyabem" - he neither performs chalitzah nor yibum.
Which type of saris are we discussing?
Saris adam - one who became a saris through human action or as the result of an injury.
Saris chamah - one who was born that way.
The Gemara explains that our Mishnah follows the view of Rabbi Akiva: the one who neither performs chalitzah nor yibum is specifically one who was a saris from the very beginning, meaning a saris chamah. The basis for this is the comparison to the next case in the Mishnah - the ailonis (a woman incapable of bearing children) - whose condition, too, was present in her from the outset.
The ailonis:
We are not speaking of an injury that befell her, but of a case where we assume the defect was present from birth. Just as a man must be capable of fathering children in order to fall under the laws of yibum, so too a woman; and since she has no capacity to bear children, she is not included in the laws of yibum at all. Therefore the ailonis neither performs chalitzah nor is she taken in yibum, and in fact yibum is forbidden to her, for she is the wife of his deceased brother, his widow.
If he performed chalitzah or cohabited - what is her status regarding kehunah?
A saris who performed chalitzah with his yevamah - "lo pesalah" - he has not disqualified her. Although a chalutzah is ordinarily forbidden to a kohen, this chalitzah has no validity, and since it has no validity it is as though it never took place at all, and she is not a chalutzah.
A saris who cohabited with his yevamah - "be'alah pesalah" - his cohabitation has disqualified her, and she becomes forbidden to a kohen. The reason: she is a full ervah (forbidden relation) with respect to him, since she is not subject to yibum, and so he has cohabited with his brother's wife where there is no mitzvah of yibum, making his act one of promiscuity.
An ailonis with whom the brothers performed chalitzah - even though she is not obligated in chalitzah, the chalitzah does not disqualify her from marrying into the kehunah.
If the brothers cohabited with her - they have disqualified her, since her cohabitation is an act of promiscuity with an ervah: his brother's wife and widow, in whom there is no mitzvah of yibum.
In summary: In this Mishnah we learned that the saris - and according to Rabbi Akiva specifically a saris chamah, who was such from the very beginning - as well as the ailonis, are not included in the laws of chalitzah and yibum, because they are incapable of having children. Consequently, a chalitzah performed with them has no validity and does not disqualify from the kehunah, whereas cohabitation does disqualify, because it is an act of promiscuity with a brother's wife in whom there is no mitzvah of yibum.