A whole body naked picture wouldn't be that bad, but close up dick pics?! What the hell am I supposed to reply with? "Nice one?" "Kinda small?" "What a monster?"
I mean, when you two have seen each other in the nude and like this kind of visual stimulation - why not, but... just after talking about something neutral on a FB messenger like nuclear fusion?! (happened to me haha).
I don't find it offensive, but if someone does that they just straight away lose all their chances of using what they've shown on me Also, I would never reciprocate with a similar picture, ever!
I occasionally moderate pics for a gay dating site so I've seen it all...
Some men really see it as an advertisement, basically showing "the goods". Maybe they think the girl's/boy's reaction would be like "hell yeah, I want it in me!"... but it's unlikely to have that effect, I think... or maybe I'm old-fashioned. I don't know how it looks like in gay community (gay men), maybe they find pics like that extremely sexy and stimulating?