You see, once you get used to the part, butt fucking is not as big a deal as it might seem.
For most guys, a tight asshole is not made for their cock, but since humans are an ingenious and curious species, we discovered early on that anal penetration can provide pleasures not only to the giving man but also to the receiving woman, if done right, of course! And after all, if there is such a thing as a male G-spot, then it is accessible via the anal sphincter.
It's very common nowadays, in many cultures and nationalities.
And be open to suggestions she might make, like maybe sticking a finger up yours in return.