Say you're in an established, exclusive relationship for 18 months.
This, right there. It all comes down to how the partners have agreed on their relationship. If you agreed to be exclusive and fuck someone else, you broke the agreement. A lot of the problem comes from societal opinion that exclusivity is an implicit agreement when a relationship begins. It does also vary from person to person, so one girl might consider a lapdance cheating while another doesn't. Reality is people need to start discussing this and negotiating the ground rules.
If I'm at a party without my girlfriend, find someone incredibly attractive and the sexual tension is through the roof, I will fuck that girl. I will also make it known that I'm in a relationship and this is most likely going to be a one night thing, but I will do it nonetheless.
For me, that has no impact on my relationship-life. I'll treat my girl just as good as I have been up until this point. Nothing will change on an emotional level.
And yes, I'm not a hypocrite. She has the same liberties. If she really wants to she can fuck whatever guy blows her mind when she's drunk out of it. I honestly don't care, as long as the vibe of our couple life is the same. And that stems from the belief that loving someone and being highly compatible with them does not, can not, and will not ever mean your attraction switch is magically shut off.
That's what I believe too, however when you say "she has the same liberties" have you actually discussed this with her? If you haven't had the discussion she may believe you have a tacit agreement not to sleep with other people.