I had an experience 20 years ago that I still think about while masturbating, probably the sexiest thing that's ever happened to me.
On a couples vacation (my husband and I, his best friend and his wife), and I found myself alone with Dave (my husband's friend) after some swimming and wine. After some innocent flirting, he confessed he'd always loved my tits and asked if my nipples were dark colored or more pink. The wine and his attention got to me so I blurted out, 'wanna see?'
I pulled the straps of my wet one-piece down and slowly peeled my suit down over my chest, exposing my full tits. The cold water had clearly worked some magic on my nipples as we both sort of gasped at the size of them.
'Holy shit' Dave said as he stared wide-eyed. Suddenly, he started pulling down his swim shorts and stroking his cock, which very quickly grew huge and stiff. 'Watch me' he whispered. Oh, did I ever! Turned on beyond belief, I watched the whole session, which ended with the most incredible explosion of cum I've ever seen. Three or four huge streams, most of which ended up on my thighs and stomach. God, what a turn on.