I'm not a doctor, but I've read some literature (papers in medicine) and some articles from medical sites
From my narrow understanding/perspective:
HIV virus needs immune system cells or cells of particular type to be able to infect and spread.
If uncircumcised:
- You have a foreskin retaining exchanged fluids underneath it
- Foreskin has cells which are susceptible to infection
> Circumcision reduces heterosexual HIV-1 acquisition in men by at least 60%. However, the biological mechanisms by which circumcision is protective remain incompletely understood.
See
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5594533/
This is also in alignment with existing STDs causing genital sores increasing chances of transmission, because if there is a sore, there is likelihood of body fighting the infection at the site, where you have elevated number of cells from immune system
However for this alone I wouldn't go and circumcise as you lose the sensation from top of your penis, it's less sensitive as I age, sometimes can't ejaculate if the pussy is loose because no sensation on the shaft and rest with a loose pussy. Myself and my family all had phimosis, had to be circumcised, it's genetic I guess.
For vaginal sex vs anal sex, you're dumping HIV directly into colon, which absorbs the material and passes it into blood stream, where you have all types of cells floating. For anal sex, recipient is at greater risk, I don't know about the penetrating party.