Husband says he feels a second “hole” when he’s inside me?

So my husband is pretty large, about 8” or 8.5” on a horny day, and girthy.

After we had sex today in a somewhat new position for us, when he was all the way in, he said it felt like his head went into another “hole.” And I could definitely feel him pushing up against something then pushing past it. It hurt the first time but then it started to hurt in a good way pretty fast.

Anyone know what that is? I tried googling what is about 8 inches up the anal canal but found nothing.