You are in an unusual position because of your age AND newness at the alternative lifestyle. I think this will work to your advantage. First of all, you are not married, especially not for many years of monogamy and/or raising a family. So it's not so much a turn-off that you want to "add" to "your" relationship the way it is with older, established couples where it will be a big change in "their" marriage. There are not the same things at stake with you and your GF. All you really need to concern yourself with is being respectful toward each other and your soon-to-be girlfriend.
I have one piece of advice. Get in the habit, mentally and verbally, of referring to your girlfriend(s) as "she and I" or "her and me" instead of "us and we". save the "us and we" for the three of you.
I'm glad you seem to have the foresight and maturity to make this work; it may take a little longer than you would like, but I have no doubt that you will appreciate it.