Erection While Being Penetrated

I usually am half erect when I start to slide my dildo in my ass, by the time I open up and accept it all I am rock hard. Then sliding it in and out rubbing against my prostate it feels like the head of my cock is going to explode, so I keep working until my prostate milk starts oozing out of my head, I like the way it tastes, its a little different than seamen. Starts out kind of thin but as I keep going it gets thicker and looks like cum.
So I keep going to try and get a prostate orgasm but once the milk stops flowing my prostate becomes less sensitive and I cannot achieve it so I jerk off with my dildo in my ass which gives me a leg shaking orgasm and produces a large amount of cum.