What causes a penis not to penetrate the vagina?

when u have an orgy ur vagina opens about 2 x the original size letting the penus smoothly go in the vagina. It depends on maybe what factors are involved, sometimes its a good idea to use Adult DVDs or foreplay to arose both partners.. good Adult DVD offer at www.adultdvdstoreonline.co.uk/home.html Adult DVD Free Shipping worldwide and low prices, maybe this will help