I am almost positive that my wife of 20 years enjoys masturbation at night as I sleep. I have been disturbed numerious times by the bed shaking from her obvious orgasium. When I ask her about it she denyes it and makes fun of me for saying anything about it. I know it is happening but can't get the proof needed to demand that she admits there is a problem.
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.