Some men see it as cheating, some don't. Some think they don't, and find out they do (mind / think it's wrong). Neither is right or wrong - you feel what you feel towards such events.
I notice that you didn't call it cheating. You avoid the word, and phrase your upset as 'she doesn't need only me
Have you asked her why she didn't consider it cheating? (that she was fulfilling a fantasy doesn't at all answer why she doesn't consider it cheating)
Looking at one angle:
It's extremely rare the married man who has never done one of the follow:
- looked at another woman and gone 'wow, she's gorgeous'
- oh I wish I could
- watched a porn movie while in relationship
....but just because they've done that, doesn't mean they've wanted to act on that attraction to the other sex (or same if so inclined). Why? (because they are committed...and committment is about values, beliefs, and actions)
And from another angle:
For for her needing more than you (or not) - it is extremely rare, over the course of the marriage, for any
person not to need more than their spouse. Why? Because there must be something to override genetics (the physical attraction we feel to the other sex).
Genetics does not
switch off when we find a partner. The attraction we feel to the other sex always exists...unless other influences stop it. Those can
(there's an emphasis on can
- love (which can preclude attraction to others)
- beliefs (which can lead to avoiding thoughts of others)
But does not include:
- committment (which, as one part, is a dedication the other partner, despite attraction to others)
If you read between the line on 'love' precluding attraction to others, it usually only occurs (and not always) when in the 'in love' stage. No marriage has that for the whole duration. When it doesn't, other things are needed.
The other things lead us to not consider / not acknowledge / avoid our attraction to others (which usually means the attraction never grows), and so, dedication our love and devotion to the one partner. I don't know that anyone can say for certain that it ever truly stops a person being attracted to others.
All that blather (and it can be gone into in a lot of detail) basically means that 'she needs only me' rarely, and perhaps never exists throughout the entire duration of a marriage. What 'she needs only me' usually consists ofi is 'what love she feels for her man' combined with her beliefs / values / fears/ actions etc.
I hope it helps some with perspective / understanding, and that your time here helps with alleviating some of the pain. Personally, these sorts of things should be dealt with through a counsellor, who can throw questions at you (though psychology is as much art as science, which means there are plenty of poor counsellors / psychologists out there)