Fact: I go deeper in her than anyone I've ever been with (although I was hitting cervix last night and neither of us liked that, lol). Maybe when she gyrates as I back up, she catches the ring and pulls it a little?
This is exactly what I was saying. She may also squeeze as you slide out to go back in which I'm sure (from experience) feels great, but that is what is pulling it off.
As far as someone else said above with precum making it slippery and such, a condom should fit snug enough where it's only getting the head of your dick "wet" with precum. It should be snug enough that it doesn't work down the shaft making it wet enough to slip off.
You may have to do either of two things if you cannot get the condom to stay on and you do not want a pregnancy. One would be do not penetrate her balls deep (if you are hitting her cervix you are going balls deep man) and the other would be to ask her if she would not squeeze you on your withdraws. Both of these solutions suck because balls deep feels "so damn good" and having someone squeeze as you withdraw also feels "so damn good".
I personally think what is happening is that inch of so of shaft that is uncovered is getting past her outer lips (inside of these lips) and the outer lips "grab it" on the way out. In the heat of the moment you may not even realize that it's slipped off until after you cum or stop. Most of the time it's after you cum and you withdraw and it's gone and you think to yourself "oh shit"!
I have never had sex with a girl who uses "female condoms" but perhaps those would work better? My guess is your size may knock it out of place but I honestly have no idea.
With the pill, what is needed to do is after she STOPS taking the pill, if to use condoms for a couple of months until the "drugs" from the pill are completely out of her system.
Too bad you are having trouble in a way because from the sounds of it, she could make me str8 man!
Have fun!