I'm having problems kick-starting my poultry industry, and searching I found that sometimes animals won't breed due commitment issues. Great. But while I was checking them, I found some males with x's besides the gender icon (x♂x). What they mean?
The x♂x males are castrated. They won't contribute much to your expansion.
As mentioned in the other answer, males with x's surrounding their gender icon ( x♂x ) have been castrated, and will no longer be able to reproduce. This could have happened before they arrived in your fortress, if they were brought with a migrant wave, although you have the option of castrating them yourself as well.
Additionally, the "commitment issues" that you mentioned are usually due to the male being asexual or homosexual (or bisexual, but not currently committed to/mating with a female). You can view your animals' sexuality using DFHack or Dwarf Therapist. I know it sounds brutal, but my non-reproducing asexual/homosexual/bisexual animals are usually the first ones sent to the butcher's shop when my food stores start to dwindle.