Why? You can't get her pregnant up the azz..........up the bum no harm done...
If you're going down on her, lightly get some puzzy juice on the anus.....even if you're fingering her, lightly brush the anus with puzzy juice on your finger........guage her reaction.......
Get her doing reverse cowgirl and play and squeeze with her ass cheeks.......not met one chick who didn't love this position or who didn't love her azz being played with at the same time, gently spank her azz, gently introduce a thumb on top of the anus......rub her puzzy juice into it...........
.....slowly introduce azz play and gauge her openness or resistance to it. No point getting over excited and trying to ram your c=ck straight in with no warning or no warm up or no lube........
.....If she'll take your thumb, she will take your c=ck........