With all due respect, @takeme0uttonight - unless your ex was one of the very few (statistically) members of the population who were born with an intersex condition (or Disorders of Sex Development "DSDs" as they are otherwise known), then they weren't born in the wrong body, as there's no such thing.
If however they were born with DSD, and were (possibly as I suspect, owing to the language you use) "categorised" wrongly - I.e., indistinguishable genitalia which was categorised and purposely developed to be female, when it was later discovered that they had male chromosomes and/or male internal reproductive organs, then yes, they would/could be stated to be living in the "wrong sex" body. However if that is the case then your argument is disingenuous, because they wouldn't be classed as what we know as trans - they were born with a medical/genetic abnormality, which was being medically corrected.
If what you are claiming, they were simply born female and transitioned to identifying as male, then no amount of transitional surgery/hormones or otherwise will change their skeletal form, which would, despite what you say, be detected at autopsy or examination by a forensic pathologist as an adult human female, and that is if it was only the skeleton remaining and they didn't also have internal organs to go by.
Doesn't matter how many people can't tell that they weren't born male by looking at them in person, or on a tv show, doesn't change that they are biologically female and always will be. And it's not wrong, or nasty or small minded to say so.
Also, and I apologise for the length of my post, but from the NHS website, no hormonal or medical treatment will be undertaken on a child before they are at the very earliest approaching puberty, far less at age 7/8 (psychological and psychiatric input will however be provided):
Hormone therapy in children and young people
Some young people with lasting signs of gender dysphoria who meet strict criteria may be referred to a hormone specialist (consultant endocrinologist) to see if they can take hormone blockers as they reach puberty. This is in addition to psychological support.
And yes, it is degrading and insulting to call other people's male partners derogatory names, in spite of the number of people moaning about their "feckless men" on this site. The minute you start calling people's partners "small dicked", or whatever it was, you've lost the argument by highlighting your immaturity.
There's really no relevance to why your partner is now your ex, it adds nothing to this discussion, so I don't think my gas is going to be at a peep any time soon.