&gt;Do people just feel a need to have some part of their interpersonal relationships be separate from their SOs? And if yes, then why?

Yes. You can still have a healthy and be mutually respectful to each other in the relationship without having to share everything that your friends or family share with you in confidence.

I sometimes share really personal shit with my best friend wrt female bodily functions/malfunctions and personal health issues that can be embarrassing and certainly would not want her husband to know all about it. My best friend personally would immediately delete those messages and text where I tell her all my shit so that there will not be a situation where her husband accidentally sees the shit I share with her about me that is only meant for her. Because of that, I do feel comfortable being very descriptive about my problems with her almost as if I was to my own doctor.

I personally don't think that being in a relationship means that you lose the capacity to keep some aspects of your interpersonal relationships separate from your SO. I certainly do not want to know all about my SO's friend's embarrassing health issues either.