It's interesting to see this discussion come back to life, with a wide range of opinions.
I agree with Prophanti in that any degree of sessioning that your significant other deems cheating is cheating, whether you're dating her, living together in a committed relationship, married or any other arrangement.
Some SOs will consider simply looking at photos or chatting/webcamming with another woman to be cheating. Others who understand your appreciation or fetish for muscular women (or strong women or fill in the blank...) might be supportive or at the very least accepting of you engaging in some behaviors around sessioning. Some might be ok, even give you their blessing to session for muscle worship that doesn't involve you touching the other woman, or happy endings or any other sexual things she does to you. Some might be ok with a happy ending but not full sexual activity. Some may sit on either side of the nudity vs clothed discussion.
I guess keeping it a secret is one way to avoid having it declared as cheating by your SO, even if you consider it cheating and are OK with it that way yourself.
Every woman and every situation is unique. Definitely something to proceed with caution about. I'm not even sure what my answer would be.