"How can a fact be evidentially untrue?"

What fact?

That "The difference between general population UK, effectively zero prevalence, and the MSMs and African born, is to do with their practices."?

That isn't a fact, that is a judgement call.

One which I questioned on the basis of evidence concernng other STI's.

These were not contracted as a result of safe sex.

5% of Gay men have HIV.

10% of young hetero women and adolescent girls have chlamydia.

In both cases, the practice which resulted in infection was unsafe sex.

So why are Chlamydia, Ghonnorhea, Syphilis, HPV etc more prevalent in young women, and why is HIV more prevalent in gay men?

It is an intriguing paradox for someone trying to apply a simple formula of "different sexual practices = different risk factor"

I call it bad luck.