How is "accidental anal" possible?

Ladies, how is it that your partners mistake your anus for your vagina when they're sticking their penises inside you?! I read about this happening and wonder if guys are being truthful when they say it's an accident. The vagina and anal openings are situated slightly different and aren't the same size. How does a penis just slip right into a tight exit like the vagina - especially if there's been no preparation to penetrate it? I'd feel instantly if my husband's penis goes hear my butthole and not my vagina...thanks for your thoughts!