A medical professional uses a speculum to dilate the vagina during an OB/GYN exam. The vagina or anus also dilates during sexual activity to prepare itself for penetration.
While the dilation of the vagina is generally involuntary, people who suffer from a disorder such as vaginismus may find that their vagina remains closed even at times of sexual arousal. Vaginal dilators of various sizes may be useful in helping these women develop better control of their bodies and learn to dilate their body when they want to. In a similar way, people may use butt plugs or dildos to dilate their anus to make anal sex more comfortable.