Well, depression could be classed as a mental illness, i suppose - but being physiologically able to have childen but choosing not to is not a mental illness, it is not a disease, and you should not think that way. Labelling what constitutes a choice as "mental illness" implies that it is a disorder, when it is not.
From what I understand of your OP, taken together with some of your early responses, it would appear that you do not have what some describe as a maternal instinct - but not all women do. A study conducted back in 1999 suggested that as many as 1 in 5 women in the UK had no maternal instinct whatsoever.
Other surveys have also confirmed that for many women, in some surveys, more women than men, thought it important that it was necessary to be financially secure and established in their careers before having a family. It also goes without saying that for many women it is equally as important that before you have a child, you have to feel the need, and be with someone with whom you wish to raise that child.
To decide you do not wish to have children is not a disease, it is not a disorder, it is not selfish - indeed, having a child that you did not particularly want would be a greater sin.
Do not beat yourself up about it, and do not let it contribute to any negative feelings you might have about yourself or your situation.
http://www.independent.co.uk/news/uk/home-news/maternal-instinct-is-extinct-for-one-woman-in-five-743042.html