Ok, this is a strange thread.
First I want to say that I would also break up with my girlfriend if I found out she lied
(but not about having done anal before, but some serious lie), because you need to trust each other in a relationship.
Speaking of trust, for most women, it is a sign that they really trust you to propose anal sex in a relationship.
OP, you got her relaxed (congrats, that is great), you took her and she felt no pain and like it (congrats again, you have skills), but you wanted it to hurt a little so you can think you're the big man?
That's is just plain cruel, even though it can be an ego-boost to be too big for a girl, I'd rather have her crawling the walls in pleasure instead of agony.
You even say you'd rather be too big and not fuck her then to do anal?
Do you even like sex?
I think you've never been turned down because of your size, otherwise you'd know that having sex is a much more pleasant feeling.
and to address the discussion about causing discomfort = you know you're big enough, I want to point out that if you cause discomfort, it means you're TOO big, not just big enough (or you didn't warm her up enough, which makes you a lousy lover).