Secret Desires

I'm in a relationship. I have been for almost six years now. Sometimes, however, I fantasize about a random guy with a big dick bending me over and pounding my ass. I just want someone to use me like a sex toy and then leave. I know how fucked up that sounds, and I don't want my boyfriend to know that, but I can't help it. I don't know why the thought of that turns me on so much, but it does.
Here's what I want to happen in detail. I want someone to come up behind me outside and get me to the dirty ground. When I'm on the ground I want him to push my top half in the dirt. Then, I want him to pull my pants and underwear down and spit on my hole and his dick to lube us both up. After he's lubed I want him to just force his cock into me while my dick is in the dirt. After that, I want him to pull my hair while fucking my ass raw.
If he had a friend with him I would want him to lift my head from the dirt and position himself under it so my only option would be to suck his cock while getting my ass fucked. Being fucked from both sides would be amazing. I would want his friend to cum in my mouth and then shove my face back in the dirt while the other guy is still railing me. Then, I want the other guy to shoot his hot load in me and leave me there with my ass and mouth filled with cum. I would even let them spit on me to show me that I was just a toy they used for sex.
I know this is intense. I honestly don't know why I have this fantasy. I don't think my boyfriend would understand if I told him this. So, for now, I'm just going to keep this to myself.