No. A lot of the tightness and pain comes from mental anticipation; if you are in the middle of vaginal sex and you are not anticipating anal, your asshole flares open and closed by itself. It is very very easy for it to slide in, and by the way, vaginal wetness is lube! Literally a natural lubricant. And if the sex is good, theres a lot of that to go around.

Actually had the exact same thing happen to me, bodily fluids are a hell of a lube. It slid right in, one stroke, and I was down. He immediately noticed what had happened and just held me while I cried from shock. I agree that the most unbelievable part of the story is that he didn’t notice. He surely should have noticed.