What it comes down to is this: Women are, of course, shaped differently from men, particularly in the hip area. But they're also a little narrower in the shoulder and, all things being equal, are typically shorter. So women's sleeping bags, such as the Sierra Designs' 15-degree Diamond Spring ($199; www.sierradesigns.com) are cut to reflect thatwider in the hips, narrower at the shoulder, shorter than a men's bag. Makers of women's bags also often add a little insulation to the torso and foot area, as women can get colder in those areas more readily than men.
So, provided the length is OK and you aren't a guy with shoulders that extend three feet from point to point, you'll do fine in a women's bag! You might even benefit from the experience, emerging from your fine night's sleep as someone who is better at cooperating with others, more nurturing, and more willing to listen to other points of view. A better human being, in other words.
For sleeping paraphernalia that crosses the sexual divide, check out Outside Online's Sleeping Bags Buying Guide.