Like you I am male
I've been where you are at and I felt like it was a betrayal of myself, and viewed it very negatively.
I don't anymore...
My advice, instead of viewing it negatively, why not help her come to orgasm. If necessary learn about female masturbation techniques, then use those techniques on her...
Sometimes all of us need extra sex male or female, and sometimes we don't want to bother our partner, or go to the extra effort of 'hiding the sausage'...we just want a bit of pleasure and an orgasm.
You can pleasure her, and you can both pleasure each other. I found telling her what to fantasise about gave me an insight into what pleases her...quite apart from the fact that it is an intensely erotic experience both masturbating your partner to orgasm and watching them masturbate also.
Please don't feel it has only negative things to offer as you could lose out on so much.
This will only continue to be a problem if you make it so.
Ray.
And this is a problem exactly how?
I assume this bothers you because you think she is doing that instead of enjoying you..if so, that is what you need to talk about.
Of course she is going to deny it. Don't you think for her that would embarrassing to admit? Not a doctor though so I can't say what will help her get help or how you can prove it.