There are many nasty things I can come up with to advice you to make her realize how wrong it is. However, the real question is, do you really care about revenge/making her feel bad?
I think she pretty much said it to your face (non verbally) that she likes her ex-bf's dick (not to forget, whom she still talks to and makes jokes about) more than yours.
I think everything set aside, when the chick doesn't like your dick, you can't trust her to stay loyal to you all your life.
I'd say stop taking shit seriously with her and just have fun. DO NOT LET IT WORK OUT!
You're going to find yourself troubled with insecurity and all such problems all your life.
If you have to make her feel guilty about such a thing, you're whipped and you're a pussy!
The dildo is symbolic. If she uses that dildo with you around, she'd fuck that guy instead if she got the chance! Isn't it really fucking clear?
There's nothing justifiable for such sort of an action!
I understand it's your girlfriend and its difficult to think of her in any way other than an angel because of those cute eyes and hot bod but think of the meaning of all of this!
She likes it better when she's fucked by a dildo that's shaped like tom's dick than being fucked by you! She'd exchange the dildo for the real thing anytime!
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