"Always" is a scary adverb because there are as many female body types as there are bike frames. If youre 5'9" with a 33" inseam, like me, you could probably get away with riding a men's frame and be perfectly happy with the fit. You dont need to settle though. In the past few years, a lot of bike manufacturers, and especially companies like Specialized and Orbea, have invested a lot of time and energy into developing womens specific road and mountain bikes.
Because women generally have longer legs and shorter torsos, Specialized uses a womens-specific geometry that has shorter top-tube lengths. And, since we generally weigh less than men, Specialized also uses lighter and thinner tube sets on their womens frames. The company also outfits bikes with gender-specific components. Their Body Geometry Jett" womens saddles, for example, help reduce pressure on your tender parts, while the brake levers on their womens specific handlebars are closer to the bars, which makes it easier for small hands to shift and brake.
Sound better than the mens bike youre currently riding? Check out the Ruby Comp Triple ($2,700), which is in the mid-to high-end range of Specialized womens road bikes. In addition to the advantages mentioned above, the carbon composite frame is race ready, but comfortable enough to ride in a century. Plus, it has a triple chain ring, which makes pumping up long mountain passes a bit less painful.
Another one of my favorites is Orbea's carbon Diva ($3,915-$6,504). The price might make you cringe, but the womens specific design rides like a dream, and the svelte black-and-pink frame will make you want to hang it up on a wall and call it art when youre not out riding.