Your girlfriend will never be adequately happy unless she can have lovers of her choice after marriage. She has admitted to you that she is cock hungry, so accept her right to have an exciting life, in alignment with the powerful sex dreams she has at night.
If you wish to take her for your bride, you need to ACCEPT mind-training for a cuckold marriage complete with contract of your duties. so you can help her GET WHAT SHE NEEDS. If she is happy, her happiness will spill all over you and make you happy too.
If you try to change her, your life will become very miserable because you are making her feel ashamed of the sexual desires in her heart - you are forcing her to "toe the line and suppress her real self".
The only reason you don't like her being slutty is because someone told you when you were a kid that it was wrong to have sex with more than one man.
The problem is that a man can rarely satisfy a woman's sexual needs on his own. She needs several guys to make her feel loved, desired and satied.
The Law of Genetics is a powerful primal urge for a woman to have babies from different men, so that her genes can be passed on to future generations. This gives the maximum chance of her avoiding bad disability genes from one man ending the family line. (go forth and multiple stuff).
Have you ever noticed how women dress up to try and be the prettiest at a party, even though they are already married. A woman naturally flirts with the best male in the room even though society demands she does not commit adultery. Primal urges keep her on the lookout for a better male (with superior genes) to make her preggy again. She knows she should remain loyal to her husband, but her emotions are saying, "mate with this new stud now". If two women have the same frock on, they become temporary enemies and the night is ruined !!!!!!!!!
Never try to change a person, just try to help them achieve all their hopes and dreams spectacularly. A large percentage of marriages fail at some point because of not helping each other to live life to the beat of the body gene drums. One day things turn to custard, because there has been too much compromising with needs not being met.
Don't marry your girlfriend until you are absolutely DETERMINED to TREAT HER her like a PRINCESS in a polymory/monogamous marriage. You must learn to treat her lovers with the greatest respect as they will add value to your marriage.
If you insist on her fidelity to you, then you MUST WALK AWAY from your relationship this week, and let her cast her line over the other side of the boat to hook one of the other 3.3 billion males in the world - many of those will be able to give her the life she deserves and craves for.