Girlfriend and her ex

[Wakeup]

She did not say she wanted to go to dinner with the ex. What she did was she asked OP what was wrong with going out with an ex because she thought it would be okay to do that (until he told her why it wouldn't be okay.)

That does not mean she wanted to go out with the ex that asked her out to dinner. They (OP and his gf) discussed it and he gave her reasons why he thought it was wrong.

She DID NOT want to go to dinner with him because if she did, she would have gone because she didn't thing anything was wrong with that until she spoke to her bf about it.
People have different rules and relationship boundaries and it is up to each individual couple to talk about boundaries so that you both know what is and isn't acceptible to one another. They did that. She didn't go. He got upset just because he asked her (she has no control over what he does only what she does) She asked what was wrong with it. He told her. case closed except op won't let it drop.

OP is paranoid and he needs to get over that paranoia or he will self-sabotage his relationship.