The secret will eventually come out
Everyone has different circumstances but I'm with MuffinBuffer. I hated trying to keep this a secret and eventually you'll get caught out. It took me literally 10 years to gradually drop hints about the Hobby. I used to retell a story to her about going to a MP. The first time I told her it was just a massage and everytime I would retell it I would add a new detail until I made it to the FS part. I also started to take her to upscale strip clubs etc. By then it was no big deal for her. Then I mentioned select other encounters. Still no big deal. Now when I am in Montreal she logs in to in Internet and picks out girls for me.
The secret is to have a strong relationship, not let up on the sex, don't try and tell her as some dramatic confession - its just sex afterall. Finally (and this is a tough one) you cannot tell her she can't go out and find some fun. It doesnt mean that she wants to, it just makes it fair. Also you may not like to admit this but while you've concentrated on keeping your hobby a secret you've not noticed that there's a good chance that she's had a couple of one-night stands when you weren't around that she's slightly guilty about. That might work in your favor and she'll go easy on you when you tell her