Situations like that are quite simple. You need to make a decision like that BEFORE you even go into a relationship. You want to fuck every girl you meet, make sure to get it straight that you are not looking for relationship, but sex only. Simple for you, simple for your girl. You can choose to be friends with benefits, you can choose to start a relationship, but make it open. There are different options. If you look for a love relationship though or an exclusive girl without love, it can be done too, but make it straight right from the beginning, so when situation like that happen you will know what to do and you won't need to lie about anything and feel guilty about anything.
If you are serious about this girl, you won't be considering messing with some other chick that might or might not like you.
If you are in a relationship with a girl just for the sake of it and just to get sex from her, than you are simply an asshole, wasting your own time and her time. A lot of men are in this kind of relationship, they become girlfriend/boyfriend just to get sex, telling girls they love them, but they don't feel anything and they cheat on them like it was nothing.
Cheating is pretty much for people who don't respect what they want. If you know what you want and you make a decision you don't need to cheat. Your decision can change in some time, sure. But, be honest to you and to people around you and you won't be doubting what is going around you as you just did. Being honest is making things simple and life is complicated enough already.