I can't say for sure why your mom doesn't like males. BUT, I can tell you why my mom didn't (my Buddy has changed her mind). My cousin's toy poodle had puppies and she gave me a black boy (I named him Bambi) for my 3rd birthday. At that time, it was less common to nueter than it is now. Anyway, my mom couldn't take the fact that this tiny little dog was lifting his leg all over the house. So, she found him a new home.
Please keep in mind this was 35 years ago and most people (at least where I come from) didn't spay or nueter.