I don't even think you know what you type, you just try to find some negative crap to say and come off as some disgruntled ex lover not making any sense acting like she owes you something.
If you can't use your imagination and need to see the inside of some girls asshole to get off, then Nikki is clearly not the girl for you. I'm not sure why you are wasting your time with her.
This is classic Nikki teasing with plenty of slips and plenty of room for your mind to run wild. If you can't appreciate it, simply move on.