You should never force anyone to do something they aren't comfortable with. Hell I was a virgin bride who insisted I would NEVER do oral or anal for my husband. It seemed dirty and something I would never want to do. After a while during oral for me I suggested trying 69 so I could also try oral for him. Hubby switched positions and as it turned out I liked doing oral for him. Some time later I requested that he slowly pull out of my pussy and switch to my asshole while fucking in the doggie position. Hubby was very patient and with a little time I actually got into the rhythm of his fucking my asshole.
So give her time. She will allow what is comfortable to her. Look at me now. I love being airtight with cocks filling all my holes at once. Maybe with time she will enjoy it too. In the mean time maybe use some small dildos or vibrators up her ass as you are fucking her. I know she will find that pleasant and exciting. Hopefully in time she will be more open to doing anal with you and who knows maybe one day she will try being airtight and find she enjoys it as much as I do.
You can always have her contact me directly at my email address at email@example.com and I would be happy to discuss it further with her. Good luck my friend just be patient with her. Enjoy whatever sex she is happy with in time it grows into much more.