One of Tolkiens appendices touches briefly on Dwarvish gender. IIRC women were slightly less common than men, and many of them weren't interested in having kids. So only 1/3 of dwarves are women and only 1/3 of them ever reproduce. So even though they have long lives they barely maintain replacement numbers. He also says that both men and women have beards and outsiders can't tell them apart. I think Gimlis line about it in The Two Towers is a direct quote from this section.