You're jealous. Do a tag search on jealousy here to read many other peoples' experiences on how to deal.
Speaking of Neanderthals and books, I highly recommend a new and very popular book called Sex at Dawn, about humanity's long history of non-monogamy. We've got a thread on it here if you want to hear some reviews before reading it. The other book most people new to poly get insight from is called Opening Up.
<rant-warning>
I have
Opening Up on my Kindle, and I have sadly only gotten to read the jealousy-chapter, but I will read the whole thing.
I did some self-cognitive therapy today, and I will share it, understanding the risk it can have for me. I created a simple scenario, including most elements that trigger discomfort in me, like where my partner is massaged by her boyfriend, rolling over, having her breasts massaged with oil, nipples kissed, her moaning with pleasure, arching her back, then them staring deeply at each other, saying "I love you so much....". I wrote this longhand, so it sticks properly. Immediately I got a tension in my body, so I seized it, grabbed it and pulled it out. Fear? Kinda, but not quite. Fear of her leaving me? Not over this scenario. Envy? Partly. After letting my hand go free and just write, I immediately actually started crying when I wrote the word "unique", so I was on to something.
Vanilla is my world, literally. I didn't know how ridiculously attached to her I was before this started happening, nor did she. How important every touch, every kiss, every caress actually was to me. For every step they take in their relationship, every touch, every next move, touching of "new" areas and so on, I feel my own uniqueness fading. I realised just today that I have tied up all of my being, all of myself in my partner, and that uniqueness has defined my self-worth. And for every loving caress they exchange, I fight not to lose my self-worth, yet I feel a little bit slips away every now and then. And therein lies my biggest job ahead.
1. Learn that my uniqueness STILL IS unique, because she's not doing it again with another me. It doesn't replace me. I do that myself.
2. Learn that my self worth is NOT tied up to the steps we have done, but if anything, what we have become, what we are today. That we have such an amazing relationship where she trusts me enough to let herself feel this for someone else.
But I am also actually green with envy of the stage they're at, the first touches, caressing body parts, exploring, enjoying the thrill. There is *definitely* a big part of envy in what I feel.