My fantasy isn't really grounded in reality, but here goes.. (warning: very long)
It would start off like me saving/rescuing a hapless young woman (16-25), petite-ish yet buxom, from a mugger/rapist in a dark alley. I picture an Indian or asian woman, but you may imagine what you want. He flees and I offer to help her up, she goes from horrified to happy/coy, looking at me with piercing blue/green "I wanna fuck you" eyes while biting her lip to make it especially obvious. I am somehow a little naive (or acting like it) and she squeezes me tightly and kisses me passionately, her nails digging sharply into my back. As we kiss her tongue releases some form of alkali-tasting hallucinogenic spore (sort of like an lsd taste, but with organic qualities of psilocybin. I'd need to do more research to give better specific details, as I'm a novice ethernaut).
When we stop kissing I am definitely affected by the chemical exchange. Walls breathing, Rothko skies, geometric figures in the bricks of the building pulsing. She keeps transfiguring appearances; film noir hottie with garters/sexy lingerie to ancient goddess covered in gold and turquoise to android-ish female (hajime sorayama/just push play album cover style woman) with cold metal skin but all the form and inner warmth of a REAL woman, etc. She grabs my very erect cock and whispers in my ear "I want you to fuck me" and bites my ear.
Our clothes disappear and we go at it missionary style on the ground; biting, clawing, kissing, wet fucking, etc. The cold pavement changes to grass/warm meadows, aikido dojos, Hindu temples, all kinds of random stuff. We also keep transfiguring forms, throughout all of history/alternate history (of our species, weird squid aliens/spore creatures/whatever!). We slowly start to melt into eachother like in Lawnmower man, only ascend into complete energy until, after an undeterminable amount of time, we come together.
We lay in eachothers' arms panting, sweaty, scratched, etc. still afterglowing from our oneness, the biochemical rush of our own sexuality/her tongue chemical. We lay in a warm bubble of light in the void of space, surrounded by stars and a nebula. She caresses my face softly and says her goodbyes, almost crying. A comet passes through us and I'm carried back to earth. I wake up clothed on the ground of the alley. With no real explanation, and I find my way to a warm cafe where I can sip some warm coffee/bailey's to ponder "what next".