because (in some cases) they want to know that if you like them that you like all of them.
Honestly even big sized guys can think they are small.
(in other cases) they want to know if you will want / can handle their size and more times than not the size of their ego or lack thereof too.
then yeah there are the dumb ones who just default on their penis cause hormones or their mental connection to their physical connection is stronger than their reasoning skills or their lack of ability to give a **** about anyone who isn't as sexual as them.
Not even half of all guys alive are as dumb as half the population of earth probably thinks.
As a man, I hate to say this, but men are simple, idiotic, sexual creatures. They will show their penis to anyone they find attractive, or anyone who shows them the slightest bit of attention, or even just whichever random people they come across.
Sad thing is, a lot of men do. That's how you tell the difference between one that is actually worth getting to know and one that you would be better off ignoring.
They are only proud about the only "interresting" (lol) they have.....
I would like to know the same of them sending pics to non transgendered girls...I mean hell you've seen one you've seen them all. Sure they all look different but the guys who send pics of theirs often...umm...don't have much to brag about. Guess they're just needing a confidence boost?