Can't say there's only one thing that does it. Women always ask if I'm a breast man, or if I like legs or asses better. It depends on the owner, really. Or maybe it's feet, fuck-me pumps, flats or sandals. Could be a hot pink thong or a pair of white cotton panties. A summery dress or a business suit. Long hair, a short bob or purple highlights. Could be a wide mouth with a perpetual smirk and laughter in her eyes.
I remember one gf had fine, delicate hands that reminded me of a china doll. Sure, I like hand jobs, but for some reason, with her, the thought of it became a virtual obsession. I couldn't keep from imagining her hands working my stiff erection to a satisfying eruption of hot, sticky cum. It was damn-near maddening to have that thought in my mind whenever we got together.
Sometimes a particular fragrance simply screams hot sex, but if another woman wore the same scent I might not even notice.
I just never know what will do it for me next, but I always enjoy finding out.