There are many ways to go bring this up with her but the most important question is does she even need to know?! What is the point in telling her? What is the end game in letting her know?
First ask her how she feels about same sex relationships. Then how she feels about observing same sex people showing affection. Then about watching same sex couples having sex. Then has she ever had an experience or inclination or thoughts about experimenting with a same sex experience. One step at a time testing the waters before jumping in.
I couldn’t ever tell my wife. She wouldn’t understand and it would be a deal breaker for her. So the tact I’ve taken is to meet married guys in the same situation.
I would buy a strap on and tell her to wear it. When she asks why just start sucking on it. Then tell her you love her but you just want to suck a dildo to see how it feels. Now if you tell her your gay or start sucking dick then you got a problem. You can have your pie and eat it too if you are smart. Women love their husbands. They see them as manly. You can probably get away with sucking her strap on and not lose face.
Talk to her, she may be into the idea of a MMF theeesome and so if so you all win. If you tell her you are into it then she may understand but if you have been doing this whilst with her and hiding it you're probably not going to be together much longer anyway.