Agree with bigsi.
My staff half traditionally been women of "child bearing age" and, regardless of whether they are single or not, they can be a nightmare.
Sweeping generalisation i know but, with a man (or a single non-parent woman) the only person's health that is an issue for the employer is the employee. With a working mother, you need to factor in the kids' health, and the childminder/carer. It can be incredibly frustrating trying to remain responsive to your customers and at the same time appreciating the plight of the employee. I often find that the other staff get lumbered with extra work and that can cause resentment.