I'm gonna say this: when I was with my ESFJ ex I increasingly got the feeling he wanted me to be more of an SFP than an NFP. There was something he loved about me, like he would be like "you can be so cool sometimes and you're so sexy" but he wanted a more ...Sensor ...version of me basically. He wanted a freaking SP and it's so obvious in retrospect.
And also, he would say things to me sometimes, things like asking me if I was going to run away with an INTx friend I have who I talked to on-line (who, btw, I've never had any sort of romantic connection to at all) because, in my ESFJ exes words "He's more the same kind of person that you are." He would also say I had a nerd fetish, and tell me he could never be "that guy."
I don't know if you have any similar strange moments with your ISTJ, but all of this is soo, soo weird to me...because ESFJ and I were so seemingly passionately, crazy in love ...and I didn't even know much about MBTI at that time...and we were basically observing type differences. It's so odd. It's why I believe in all of this theory, it's like OMG A-HA THATS WHAT IT IS.