1) It should first be pointed out that an Ailonis is a woman who clearly is not capable of bearing children. The Gemara in Yevamos 80b lists certain characteristics of an Ailonis: (a) she does not possess nipples, (b) her stomach is not similar to a woman's, (c) her voice is deep like a man's, etc.

2) The Gemara in Yevamos 65a also acknowledges that sometimes it is the man who is infertile. If there is a dispute between the husband and wife concerning who is responsible for the inability to have children, the wife is believed. This is because she knows whether or not he is capable of being "Yoreh k'Chetz" (a pre-requisite for conceiving), while he is not aware of this.

All the best,

Dovid Bloom