I'm not saying there is something inherently offensive about erections.
You are completely missing some very important pieces of information from your responses. First off, you cannot ignore the intent when talking about dick pics. We aren't talking about a man getting an erection in public or something. Yes, that happens sometimes and you have no control over it. It isn't offensive in itself. But taking a photo of your erect penis, with then intention of showing someone how turned on you are, is different. There aren't a whole bunch of men sending pictures of their flaccid penises. The purpose of the photos, and the messages that accompany them, is to show that the man is aroused and ready.
Secondly, you are completely ignoring the power dynamic that exists in our culture. There is a huge power differential between men and women. Like I said in my last comment, people aren't forcibly penetrating other people on a regular basis with their boobs or vulvas. So yes, it IS different.
Appreciation of the human form is a completely separate issue as well. I'm not offended or put off by Celtic penis statues or Michelangelo's David because they're not being used to try to have sex with me.
Your simplistic suggestion that women can/should just choose to have a different response ignores the fact that we are living in a patriarchal society where rape culture is prevalent. I get it. You have no idea what it is like to know that the only reason you haven't been raped (so far) is dumb luck. You don't know what it's like to be constantly objectified and discounted because of your genitals. Do you seriously think that people are oppressed because they choose to be? That is incredibly ignorant.