A friend I'd shown interest in once upon a time sent me his cock pic via text. I was sat in a restaurant at the time with a good friend. He'd never done anything of this nature before and it was quite unexpected... I just hope the people sat behind me didn't see when I opened it.
I lost any and all interest I'd had in him in that moment.
It isn't about hating cock, penis envy or anything else of that nature. It is about respect. It is about you not assuming that at any given moment, without request, we are sat there longing to see what you've got dangling between your legs. Assuming that at the moment you send it, we are in a situation where innocently opening your text to find a penis would be appropriate or at least not cause issues.
Let's not forget that for generations, two things have been happening:
1. Women have had to listen to countless tales of how women are just out for what they can get, that they are all take and no give.
2. Women have had to listen to countless tales of men sharing stories, intimate details of their sex lives with friends.
Those two things combined leave a lot of us feeling like we're expected to send something back (though I'd guess that most on this site at least are like myself and don't really give a fuck about what may or may not be expected of us) and that if we do send something back, it'll be around your entire group of friends within seconds and all over the internet within the hour.
You send one if asked for one, or perhaps if she's sent you something first (and you feel like reciprocating) otherwise, keep it to yourself.