Medically or biologically, there's no difference. An ass is an ass is an ass.
But as has been pointed out, especially with regards to work, you are crossing over into (or have previously been active in) talent pools which have a higher potential risk/incidence of HIV transmission.
Of course, when we factor in circus acts like double anal with internal popshots, then the risk of anal tears which greatly facilitate transmission are much greater as well, regardless of the gender or orientation of the performers. Added to this, when you are going to 'developing' countries to shoot so that you can lowball your production costs, then you might as well be playing Russian Roulette with five rounds and one empty chamber.
What is bizarre to me is this: John Stagliano caught HIV after being anally creampied by a tranny in Brazil.. Ergo, Evil Angel banned the shooting of anal creampies by the directors it distributes (which is why there was much rejoicing from anal creampie enthusiasts when Erik Everhard made the leap over to JJV, where no such restriction is in place). Yet bizarrely, no such edict was made about shooting trannies or shooting in Brazil.
The act in and of itself is not risky (if both performers are HIV free), but when the factors of tranny talent pool (a pool which can both pitch and catch, hence the raised risk) and third world location are figured in, the risk goes through the roof...that's why nobody is in a rush to go and shoot bareback double anal creampies in Uganda or Mozambique.
The short answer is: The map is not the territory. It's all very well to indulge in blue sky hypothetical arguments along the lines of 'All things being equal...', but the fact of the matter is that all things are not equal...there are mitigating circumstances to be taken onboard, hence the quasi-pariah status of active crossover performers such as yourself.