I personally don't believe a person should be friends with their exes if they're starting a new relationship with someone else. I would however give her a pass intially because you two were just starting to see each other and really didn't know if this relationship would lead to anything major.
Now that you've found out the whole story at the 18 month stage and all the cards are on the table. You shoudl inform her that this behavior is a deal breaker and if it happens again, your out!
What and see how things pan out, but don't marry anyone if you can't trust them.
ANSWER THIS QUESTION
This is a long answer, but I'm thinking of both sides and also addressing what might be entering into your mind (rather than just what you said in the question).
I feel it is possible to remain friends with someone after a relationship. But there is a difference between a simple friendship and being really emotionally involved with someone...being more than friends and having a deep, emotional connection. There is a big difference between the two, and both of you need to assess which it is--together--nothing hidden.
Maybe you could suggest to your girlfriend that she look at this situation and her friendships in this way--would she be okay if you had the same type of relationship with another female? If not--then she needs to modify her behavior. Thinking about it in this way could be a motivator to help her do it. Double standards are an issue. She can't do what she wouldn't want you to do.
On the flip side, does your girlfriend have other male friends (besides ones she has been in past relationships with), and is that okay with you? It should be.
A person should feel secure enough in their relationship that they don't feel like they have to put blinders on their partner. That would indicate there is a jealousy issue and jealousy is not a good thing.
In relationships we need to feel secure enough about each other's love so we can trust. Honest, open communication is really essential in these situations.
If she continues hiding her relationships, she could either be hiding because they are not truly innocent relationships, or just because they really are innocent and she knows you would be upset (and she doesn't agree with your wishes).
But hiding is not okay either way. If she wants to have male friends and let you have female friends, it needs to be out in the open and you both need to be in agreement with it and satisfied with the plan. She needs to be comfortable with you doing the same thing.
Try to really sit down and talk about all angles of this situation openly and honestly. If you can't both be happy now with the desires of your partner on this issue, it just won't work in the long run. Ultimatums really don't work. Both parties need to discuss and agree.
If you can't work it out, you'll know. But I hope you can!
ANSWER THIS QUESTION