I think I pissed my girlfriend off.

[Kes]

Girlfriend: What do you mean, 'sadly'?
Me: Well, she is pretty. It would've been nice if I did her before we started going out.

The correct answer would've been, "If I had actually been going out with Sarah, I would've regretted never being intimate with her. But since you and I have each other, there's nothing to be sad about in the least, and Sarah never crosses my mind.

Hah! That is in <i>no</i> way the right answer. That says "I wish I were dating Sarah instead of you. / You're my fallback girlfriend."

She asked you a loaded question, and if she didn't want to hear what you had to say, it was her fault. Yes, the "sadly" was tactless, but she set herself up. I personally think she needs to chill a bit.