STIs are usually transmitted via body fluids through mucus membranes &, hence, we use condoms to avoid transferring the fluids to those surfaces. Unless you have a cut on your skin, it's a pretty good barrier against pathogens. So cum spray away as long as you avoid the eyes, nostrils,, & all other organs with exposed mucus membranes. Lol
I never really understood the use of condoms in pornos -- especially when in the end, the dude pulls it off and blows his wad all over the girl anyway. What's the point?