I've seen that happen a lot with sissy porn, a lot of them don't get hard but it's so sexy watching their useless sissy nuts and soft dicklets swing and flop around as her man fucks her brains out while satisfying himself. It seems our gurly side takes over and our main need is to please and serve cock,not get hard and ready to cum ourselves like a real man would. I know I prefer not to cum when I'm with a man, it keeps me horny, needy for cock and eager to serve please, but like you, when I'm licking and sucking my hot girlfriends wet smelly pussy and cum covered asshole I'm hard as steel and wind up plowing her pussy. I guess it's because I'm bi. Go figure. 🙂