I'm a little late to the thread here, but I have to say you are in a similar situation as me. I've been with my gf for 8 years now and plan to marry her. I, like you, never had a real relationship before her and wondered what sex would be like with other people.

I'll start by telling you my GF doesn't have a sex drive either. The part where you say she "enjoys sex, but does not need it" and "she does not initiate sex, ask for it or give cues" hits so very close to home. My gf is this exact same way, she doesn't masturbate or fantasize or anything, she just goes on and doesn't think of sex. Now I should note she is willing to have sex, but like you, I want someone thats "tickles my fancy" and is really interested in the act.

Over the years her lack of initiating sex had really gotten on my nerves to the point where I nearly cheated on her with women that did want sex and that were willing to approach me for sex. It got to the point where I had three or four different girls that I was certain I would sleep with, but I never did. Maybe my conscience was too heavy, maybe they didn't stimulate me, maybe I just didn't want it as bad as I thought. All I know is that I had the opportunity to cheat, but I never did.

So a couple years pass and I pretty much had come to terms with the fact that I was going to initiate sex for the rest of my life and that it would probably only get worse over time after kids and with our careers taking off. So I did what I thought was best and just kept on with the way things were.

Couple more years pass and we have moved out of state and away from family and friends together. Times get really rough for me as my youth is slipping away and what I feel like are my chances to get with younger adventurous girls. After being away for a year Im starting feel like I want to sleep with other women again and I'm not sure if I'll go through with it or not.

Most likely I won't do anything because I can only imagine how bad it would hurt my gf if she were to find out anything happened, but just the sheer frustration of my sex life is sometimes enough to make me do something.

I guess the moral of my story is don't let these feelings linger. Either break up with her, power through it or cheat. Do whichever you feel you can handle or it will eat at you for as long as you let it. No one on reddit is going to make the right decision for you, that's something you'll have to do on your own.