I noticed you posted something similar early on. I want to share something with you that is considered immoral and/or amoral.
I am currently seeing 3 women at the same time. They might define me as more, but I only see them as FWB. I have limited them to that status for their history and style of living, none of which look remotely trustworthy.
These type of women live in a world absent of any morality that suits your traditional desires. They want to live it up as much as possible before getting fat and old, and they do so by absorbing all the male attention and validation possible. This works their manipulation skills and gives them a sense of power. These are fun women who just want to have fun, but only that. Going out of your way emotionally or financially for someone so capable of and willing to manipulate you is unwise. They will leave you eventually.
This is the direction things are moving in and becomes more common every day. In most of the world, the provider role traditionally held by men becomes increasingly difficult, as the average pay falls and the average woman is more likely to be professional. You are less needed today for your traditional qualities, and thus the traditional women begin to vanish.
What you have right now is a woman who's had many men before you, and nothing in history suggests there won't be many more to come. It's her fun and her freedom. Some would call that behavior inherently wrong, but in many ways it's only reasonable. The only reasonable adaptation on your part would be to exercise some selfish behavior of your own. Not only is it fair for you to not constrain yourself to rules they don't follow, but those rules are not rules they expect you to follow anyways.
There's a reason for the analogy that the key that unlocks all doors is a master key. The door unlocked by any key has a shitty lock. It is because men judge objectively while women judge by consensus. This is why you currently have a problem with the girlfriend who's sleeping around. However from their perspective, being a sexually successful man displays to them a man who has already been judged well by many before her. The best man is what they want most. She would feel quite different under the circumstance that you were the one sleeping around before going official. This is why they don't see it as wrong or feel any guilt. Doing so is against their nature.
If you're dreaming of having a traditional family in the future, it'd be best to attempt such a thing with a traditional woman, from a traditional family, and who has the appropriate moral values to support it.