The OP handled this all wrong from the start. You never forbid your girl to go somewhere, it lowers you in her eyes and it doesn't resolve anything. She wasn't involved in the decision, so nothing's changed in her mind about her willingness to do it next time. Now you still have to worry about her doing it behind your back and making you out to be a bad guy in this situation.
What the OP should have done is said in a calm voice; "It really bothers me that a guy you just met asks you out on a date and your first instinct is to say yes. I would like to invite a new girl on a date just to show you how it feels, but I have too much respect for you and our relationship to consider something like that. I'm trying to give you the benefit of the doubt and assume you just weren't thinking, and that now that i've brought it up you'll reconsider."
And then you should have refused to argue the issue. If she tried to claim this is innocent, you should have only one reply. "I understand you think differently, but nothing you say is going to make me think that you going out on a date is okay. I'm not going to tell you what to do, but you should know my feelings aren't ever going to change about this being acceptable relationship behavior."
If the OP handled it in this manner one of 3 things would have happened.
1. She would have agreed to his viewpoint voluntarily, admitted she was wrong, not went on this date and known to not even consider saying "yes" the next time a guy asked. Edit: She either agrees she's wrong or simply acknowledges that since it bothers the OP, it's more important to her that the OP is happy than that she meets some random guy. Either way, she's the one that makes the decison that going is wrong.
2. She would have went anyway, but felt horrible the whole time thinking about the OP being at home and mad/sad. The date would be a horrible experience and she'd cut it as short as possible and run home to the OP apologizing and promising never to do it again. The two of them actually grow closer through the experience, as the thought of losing him makes her realize how much she cares.
3. She went on the date, and told him it's his problem if he doesn't like it. The OP would know for sure that the relationship is past the point of no return and be able to end it without any regrets, because she was already gone anyway.
What the OP should have done is said in a calm voice; "It really bothers me that a guy you just met asks you out on a date and your first instinct is to say yes. I would like to invite a new girl on a date just to show you how it feels, but I have too much respect for you and our relationship to consider something like that. I'm trying to give you the benefit of the doubt and assume you just weren't thinking, and that now that i've brought it up you'll reconsider."
And then you should have refused to argue the issue. If she tried to claim this is innocent, you should have only one reply. "I understand you think differently, but nothing you say is going to make me think that you going out on a date is okay. I'm not going to tell you what to do, but you should know my feelings aren't ever going to change about this being acceptable relationship behavior."
If the OP handled it in this manner one of 3 things would have happened.
1. She would have agreed to his viewpoint voluntarily, admitted she was wrong, not went on this date and known to not even consider saying "yes" the next time a guy asked. Edit: She either agrees she's wrong or simply acknowledges that since it bothers the OP, it's more important to her that the OP is happy than that she meets some random guy. Either way, she's the one that makes the decison that going is wrong.
2. She would have went anyway, but felt horrible the whole time thinking about the OP being at home and mad/sad. The date would be a horrible experience and she'd cut it as short as possible and run home to the OP apologizing and promising never to do it again. The two of them actually grow closer through the experience, as the thought of losing him makes her realize how much she cares.
3. She went on the date, and told him it's his problem if he doesn't like it. The OP would know for sure that the relationship is past the point of no return and be able to end it without any regrets, because she was already gone anyway.