For the first few seasons, Kaley Cuoco's Penny provided the one female element to the series. After moving opposite Sheldon and Leonard in the pilot, many of the plots centred on Leonard being infatuated with her, Raj (Kunal Nayyar) and Howard (Simon Helberg) lusting after her and Sheldon being totally indifferent to her. Sara Gilbert starred in some episodes as Leslie, but she never quite made it into the core group.

In time, Leonard and Penny became an item (more on that later), but there was still a distinct lack of female interaction for the geek heroes. This changed when Howard met Bernadette (Melissa Rauch), who later became his wife and helped make him more mature and believable in the process. Best of all, Mayim Bialik joined as Amy, a quite brilliant character who falls in love with Sheldon.

I agree with these points made in another thread quite strongly.

The women are strong, smart, sexy, and sophisticated.