I'd have to agree with mplsingleguy that if a guy is flaunting his cock in an exhibitionist sort of way I'll just smile and say "nice." If spoken words or gestures have already suggested a sexual interest by the time I see his genitals I'll say "nice cock!" My two favorite comments received, however, were less direct and more imaginative.
The first was a very attractive woman in her thirties that I bumped into while hiking nude at an outdoor clothing-optional facility. We had seen each other around the place earlier and I was quite attracted to her, but we had not previously spoken. Where we met on the trail it was narrow and it traversed a steep slope, leaving little room for two bodies to squeeze past each other comfortably, so I stepped up the slope and stood on a flat rock to give her free passage. As she continued down the trail she stopped in front of me, stared at my cock (which wasn't all that far below her eye level) for an indiscreetly long period of time and then looked up to my eyes and said "We simply HAVE to get together." And that evening we in fact did, in the privacy of her quarters.
The other one came from a dumpy, unattractive, even repulsive looking older guy that I ran into out at Blacks Beach in San Diego county, CA. I was just strolling the beach nude when this guy came up to me, took a long look at my crotch and then said "you really ought to have that thing looked at." I assured him that I was having it "looked at" on quite a regular basis, thank you, and sent him on his way.
