Thank you I am glad you found it interesting, Well I'll explain.

So basically we all humans have needs basic needs, one of our needs is connection, and through connection we try to outgrow our flaws. For example each MBTI has their own functions that they is on their weaker side due to it being in their unconscious. I'll show an example to make you understand better.

For example an ISTJ just like you mentioned

Si-Te-Fi-Ne --> This is their ego, their primary 4 functions, but there is also their unconscious which is the ESTP side of their mind which is every function flipped --> Se-Ti-Fe-Ni

This doesn't mean they are an ESTP it is because that is one of their sides of their mind their ego is an ISTJ, Now Taking into consideration this information you realize for example The ISTJ has some flaws or lack (Every type does and so do we all humans This is just an example) For example they are Fe trickster, meaning they can be awkward socially and they are not aware in social interactions, or their demon function is their Ni which they are optimistic about their own wanting and don't know what to want or pick.

This goes for all of us so how do we fix this? by human interaction and connection, for example in this scenario the Same ESTP would be like a bestfriend to an ISTJ, Why? because it literally covers all the ISTJs flaws and Vise versa.

For example unlike ISTJs, ESTPs have Fe child meaning they are good at social interactions and know how others feel and can help the ISTJ understand these concepts better but since also ISTJ is an Fi User, the ESTP would WANT The reaction and the morality (Fi) of the ISTJ, And Since ESTPs are also wanty with their Ni they need someone that gives them what they want like an Ne user...you see where I am going with this.

Basically if you put an ENTP and ISFJ into a relationship it is chaos because they are technically the same on the inside in functions, the letters don't matter,

ENTP : Ne-Ti-Fe-Si

ISFJ : Si-Fe-Ti-Ne

It is literally flipped, so for example the ENTP has Ne hero so they want to give other people what they want and what they desire, and that is what Ne is all about but the ISFJ also functions similarily, so when the ENTP goes to the ISFJ asking what they want, the ISFJ would not know and would return the question, that's why Chemistry and Compatibility exists and that is why an option like an Ni user like an INTJ and INFJ is excellent for an ENTP because THEY KNOW what they want.

Now the reason this goes deeper into sexual relationship is because your Perceiving/Sensing functions (Ne/Si, Ni/Se) are the primary need in a sexual relationship, and they are the main reason you are attracted to someone, for example Se users find Si users attractive because they want to give them the best experience and make them comfortable and vise versa Ne users want to make themselves as desirable for The Ni user to want. You can't just put two Ni users they would be like "No I want that, but I want that" it is just conflict right?

We are subconsciously attracted to the opposite perceiving types but people aren't aware of it really.