I'm being a realist about my girlfriend's weight gain.
How are you trying to be a realist about your girlfriend's weight?
I know she's not going to change, so I'm being realistic about that.
Even if your girlfriend did gain weight, it is rude and inappropriate to call her out over it directly. You should try to be supportive of your partner and bring up issues in a more constructive manner.
You are not seeing what's been going on. She's made me feel like shit and that's why I'm going out with another girl.
If you're unhappy about her weight gain, you need to talk to her about it. She may very well be mad that you don't accept her at a higher weight. But just cheating on her because you're "realistic" is mean.