Friendship can be a funny thing. A true friendship should last a lifetime. However, some people find themselves being frozen out of friendships when one of them becomes a parent. One of the discussions over at s-mail.proboards.com is about friendship cut short because the one finds out the other has children. The same could be true for people without children being dumped by new parents. "How could they possibly understand the commitments and responsibilities that come with having children?" is perhaps one train of thought. I haven't myself lost single/child-free people as friends when I became a mother. I gained "friends" who also were mothers. Parenthood might turn you into a zombie at times but you are still you. You might not be as carefree and fancy-free as you once were, but you are still you. I can understand if people don't telephone you as often because you might have just gotten the baby to sleep, but with penpal letters, this doesn't disturb and the conversation can be continued at whatever time of day or night is convenient. Still, some people seem even uncomfortable continuing penfriendship on finding out their penpal does not have a similar family status.
Browsing penpal site profiles yields those who only want to correspond with other mothers (maybe with at least 2 children only). I can understand if the phrasing was "mainly mothers" because you could potentially share advice, tips and experiences.
One of my best friends here I see regularly does not have children herself (and has never wanted), although she does have 3 generations of German Shepherd dogs and she was also a teacher before retirement. How does the "you wouldn't understand" comment apply to her, as being a teacher and animal companion? She is not a selfish person.