The reactions that men most desire from recipients are positive

The researchers found that men who sent unsolicited images tended to be younger, more narcissistic, and more sexist. The most common motivation for sending such an image was hoping to receive sexual pictures in return, followed by hoping to turn on the recipient.
Only about 18% of men reported sending unsolicited images for their own satisfaction. Most men also hoped to make the person who received the image feel excited or attractive.

“People should revel in the importance of science. That is, people often believe that the main reason men send unsolicited dick pics is because they dislike women/are sexist and wish to anger, shame, or otherwise express their hostility toward women,” Pedersen told PsyPost.
“This study revealed something quite different. This study revealed that the main reasons men send dick pics are for those of transaction (‘I’ll show you mine and you show me yours’) or partner hunting (‘you’ve seen my dick, let’s hook-up’).”

“Further, the reactions that men most desire from recipients are positive (feelings of attractiveness, for instance), not negative, as people often expect,” Pedersen said.