I've done that many times rubbing my cock head in my wife's ass crack starting to open her asshole with my soft cut cock head in as my hard shaft was just starting to open her hole. There were times I got too excited starting to penetrate her asshole and shot my cum in her ass crack and asshole. Sometimes I got my cock in her asshole but I didn't last long as I spent most of my time rubbing my cock head in her ass crack.

Cum makes a great anal lube and I have ass fucked several women over the years and some were easier to penetrate. I found if I spent some time licking her asshole it was easier to work my cock head inside.