Most Helpful Guy
Even though being on the receiving end of such a pic if you're a guy works great, girls don't think the same way. Most guys think girls think the same way they do.
I wouldn't worry about it. He's trying new things, seeing what you like and don't like. Learning about you. He did apologize and gave you an explanation, which was right for him to do.
If this is a new relationship, leave him a little rope to try out new things. He really is only bouncing things off of you to see what you like and don't like, and it's great that he's willing to respect your feelings with an apology and a nice gesture of love if he tries something and it doesn't work.
Most Helpful Girl
This is my Answer here, dear, that he Told you straight out in being an Honest John that he had no problems in you saying "No," and that he still respects you and loves you and he even apologized for Wanting to 'Send me a pic of his d***?
He most likely is ready to take the Next Big step in having sex and wanted to Let you Know it by show it" and see if it would get your Seal and Squeal of approval of going to the next level by having you look First hand at His... Merchandise.
Good luck. xx