I suspect this story is a fabrication. There'd be no need to say, "This is Jackie, my girlfriend." He could easily have just said, "This is Jackie" and leave it at that.

If this couple had visited the BF's relatives for real, you can bet your ass that on the way there (if not before) he & his trans "GF" would've discussed at length how the "GF" wanted to be introduced & they both would've decided on exactly what info to reveal & hold back. The way lesbians and gay people used to do back in the 1960s, 70s and 80s.

I just know my gossipy aunts would have been sitting there staining at her the whole time, probably mocking us in their heads, transphobic bullshit like “why is he calling that man in a skirt his GF”.

Since the BF has such a negative view of his aunts - and he also has powers of omniscience so stupendous he can read his aunts' minds in advance - why would he have brought his GF there in the first place? After all, in his eyes they don't just live in a small town, they are small minded and "ignunt" too:

some of my relatives in a small town who a know to be a little backwards/ignorant in their thinking.

If this visit did actually happen, I suspect it's coz in his heart of hearts the BF hates his former BF for starting on cross-sex hormones and now pretending to be his GF and expecting his BF and everyone else to play along with the charade.