I understand why she is upset, but it is because she doesn’t really understand and is acting immature imo. Which is hard for me to say because I have def been upset over similar things. I think for a lot of women, when we are horny its because we are attracted. Not that we find someone handsome or good looking, but attracted. Its like a magnetic pull. That shouldn’t be confused with having real emotions for them, although plenty of women themselves think they have real feelings when they dont. I would say this to her:

“Hey, if you want to break up with me over this then that is your decision but I want to offer one more explanation to this then I will at least know I tried to help you understand me better. If I walked in on you masturbating my dick would probably be extremely hard and I would desire to watch and help you finish. I would feel pulled in because I am extremely attracted to you and love you. If I saw your friends, your sister, some rando female, my dick might get semi hard because it’s simply a visual cue. But heart wouldn’t be beating faster. My eyes wouldn’t linger. I wouldn’t feel pulled in. I wouldn’t take in how they looked or care to remember. Its the equivalent of a random shiver. Being semi hard in of itself doesn’t indicate attraction. I can get semihard if the wind hits me the wrong way. Fuck, on seinfeld they made a joke about george freaking out because he got hard from a guy giving him a massage. As a guy, sometimes my dick get somewhat hard out of my control. There are many reasons why my dick could start getting hard that dont have to do with sex. It doesnt mean I want to act on it either by sex or masturbating. I am in control of my actions and regardless I dont want to do those things”

Def clean it up maybe shorten it but you get the jist