Guys come on women's faces all the time. I suppose it gets into their eyes every now and then.
I have had it on my eyes but it never got into them or irritated them in any way. I have actually never heard of that.
If it did get into a woman's eyes, I bet it would result in nothing irritating or painful. Semen is very benign. I am very sure it would be like water in your eyes but so much thicker it would be hard to even get there.
Why are you asking this? I am sure it is extremely rare.
It won't sting if guy keeps his body hydrated with adequate intake of water. I saw a girl get sprayed on the face from a jerk circle of seven guys standing around her and all trying to cum the quickest over her. I thought it was gross, they were all drunk.
Speaking from personal experience; it stings like hell! That's how I ended up with 'pink eye' My eye looked bloodshot and weird for a few hours... but meh, there are things that are a hell of a lot more painful.