I'm not so high in polyamory experience, but let me share mine.
My husband is only 7 years older than I am, but he has always had a lower sex drive than I have. We married when I was 18--a lot of people have criticized us, saying I was just a baby and that I didn't know what I was doing. I felt, and still feel, that I was pretty mature; I know that I pursued him, so I don't think the term "robbing the cradle" applies. My husband, by the way, feels bad that I "missed" my youth, but I kind of feel like I was youthed out by the time we had kids (I was 22). It makes me crazy angry when he assumes that I don't know my own mind because I was so young when we married.
I was able to cope with the disparity in sex drives until recent years, when I apparently hit my sexual peak. I admit that I thought often that adding another man to the dynamic would even things out. But I had been raised "one marriage, one partner", AND we had friends with an "open marriage" which they called polyamory but I'm learning wasn't really, and they ended badly, so the thoughts stayed thoughts until I kind of accidentally fell in love with another couple. Whom I have not had sex with, btw. The side effect from all of the talking, talking, talking we've all done with regards to our relationships is that my relationship with my husband has improved. Turns out that talking improves some things. Who knew? And sex without love is unappealing, but now that I have feelings for couple #2...it's amazing how I feel when I'm with them.
So I guess the moral of my story is...
Don't assume you know what she wants. Talk to her. Make sure you know what you want, then communicate it clearly. You're probably going to end up compromising anyway. Please don't discount the idea that you can have a good, nonsexual relationship with another member of a polyamorous relationship. And I haven't been at this for long, but again, I'm completely amazed at what a wonderful thing this new polyamorous relationship is. So yes, it can be a good thing.