That's a hard no.
My husband and I returned home after a night on the town and we were getting it on, We were doing well and I had the pleasure of being in my favorite position, Missionary with my legs up on his shoulders. Usually he grabs and gropes my bum which is always a nice bonus for me as well as him but last night he was doing his thing and I noticed he was getting a bit close to the wrong hole with his fingers and it is not uncommon to have a finger stray off course and touch it by accident but I never say anything and he always gets things back to where they should be but last night I felt his finger brush over it and then all of a sudden he shoved it in, Just his pinky apparently but...Yeah, Not ok.
I gasped and said "Nope, Nope, No, take it out", But just then he started coming so I took one for the team and let him finish, He was looking me in the eyes with his finger in my bum and his p**** in my v***** while he came, I was so weirded out and I know I was not making a very pretty face as it was happening but he was moaning "Good god that's f****** hot", So I have sperm running down my butt crack and he starts fingering my bum and I didn't mean to but I gagged and he stopped, I looked at him and said "Ok, I love you but if you don't get your finger out of my ass..." and he started laughing and pulled it out.
I made him wash his hands and when he got into bed I said "What the h*** was that", He shrugged and said "I just thought I would try it", I told him "No, Don't f****** try stuff without asking...F*****", He thinks it was funny but I told him it ruined a possible O for me and left me unfulfilled and I had to get up and go take care of myself in the shower after he fell asleep because I couldn't fall asleep being left hanging, I explained that it is unacceptable to shove a finger in my bum at any time but especially without asking.
Should I have to explain that to a guy I have been with for almost 20 years, Come on.