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It is true that the anal canal is tight in females (in both sexes actually), but it can be distended if required. For example the anal speculum and colonoscopes are two instruments widely used in medical practice to visualise the rectum and anal canal. They have pretty big diameter and can be used in individuals without any exposure to anal sex if used properly and with lubrication.
There is a sphincter called anal sphincter which can cause pain if it resists the dilation. But if the penetration was direct with out much stain and with lubrication it can dilate without causing much pain.
Bottomline is that, this episode alone is NOT an evidence to say your partner has previous experience in anal sex.
Hope this helps
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