I think Ceoli made a couple good points for you above and I'll try to add a little helpful clarity if possible.
I seems to me that there's a lot of misconceptions out there regarding what "polyamory" really is - in both theory & practice. It's complicated - yet simple.
It's my guess (and only that) that the majority of people who find themselves involved with a variety of "poly" (meaning multiple) lifestyles got started down that path from a "sexual" jumping off point. Exceptions apply of course but my suspicion is that the majority got stared that way. Sexuality is a big deal...in human nature, in culture etc.
Right now - you seem poised on that launching pad and are about to take off - but in WHAT direction ? A lot of people like to stand polamory on one extreme and swinging on the other. We tend to drape a net between them to also catch all the possible variations in between. At some point you'll discover where your ship lands - on one end or the other or maybe in the net.
But here's where we see a critical point of understanding you will have to search YOURSELF for.
It appears you are launching from a primarily sexual platform - pretty common. The question now becomes - if in the course of pursuing this path you discover that there is an emotional element enters i.e. some true bonding (love in all it's various variations?) - how will THAT feel to you ? What will it mean to you if/when your GF professes some deeper connection to another person - be it male/female/TG whatever. Or what if it happens to YOU ? How will you both feel about that ?
It's a good discussion to get out on the table right at square one. And keep in mind that your views at a given moment are subject to evolution
But having the facts and possibilities in front of you are critical to building strong relationships.
Hope that helps some.