SUBREDDIT: r/relationships
TITLE: I [M35] said something to my girlfriend of 1 year [F31] last night. She's upset even though it's a complete misunderstanding.
POST: My girlfriend and I have been working out a lot together and last night I was checking out my progress in the mirror. She jokingly said that I was going to leave her for someone better (which isn't possible) now that I've got muscles growing. I was going to make a joke about how I'd dump her after meeting someone at the gym (we always joke like this). I refrained from making that joke and told her, as I always tell her, that she's perfect and that I couldn't imagine anyone possibly better. 

In the next breath though, because I had been thinking about making the joke about the gym, I remembered that I had seen a mutual acquantince working out, too. My girlfriend has referred to her as being the prettiest girl in town. While she is certainly attractive, there is absolutely no contest!

Unfortunately, in the next breath, I said, "oh, btw, did you see X at the gym tonight?". So, my girlfriend got upset because in her mind, when she said I would leave her for someone better, I must have immediately thought about X. Ugh. 

I don't think I did anything wrong, it's just a really unfortunate thing to say and now she's upset and thinks that I'm full of crap when I tell her how beautiful she is and how lucky I am. I've always been honest with her about my feelings and it kind of kills me to think that a careless remark like this could hurt her so much.
TL;DR: 
Girlfriend and I joked about me finding someone better. I unfortunately mentioned right after about seeing one of our attractive female friends at the gym. Now she thinks that I must think the friend is more attractive than her. 