Evolution has bestowed equines with telescopic vaginal muscles.

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Evolution has bestowed equines with telescopic vaginal muscles. What this means is that their vaginal interiors can enlarge or contract as they see fit, even to the point where a human finger cannot breach their bodies if they do not will it. What this mean is that all the hentai and the fanfics about ponies being lewded and raped against their will simply could not happen if the mare was in good health.