In heterosexual vaginal or anal sex, if other factors are equal, a woman has a biologically higher risk of infection compared to a man.
The surface area of the vagina or anus is far greater than the surface area of the penis.
The risks of tears to the vagina or anus during sex is greater as the these inner linings are more delicate than the skin on the penis.
The length of time of exposure of the vagina or anus to semen is greater than the time the penis is exposed to the vaginal fluid or rectal mucosa.