Men and women cheat for VERY different reasons (because men and women are wired very differently), and it is very important to understand this
Men are biologically driven to want to screw as many women as possible, to spread their DNA. This is why a lot of men cheat - to a certain extent, it is programmed into them. (Obviously - that's not an excuse, because as cognitive animals, we can CHOOSE not to cheat, and that is what trust, loyalty, etc is all about.) As such, men are attracted to ANY women that look healthy and of breeding age.
Women are attracted to men who can provide for them and protect them (from the same biological reasons). They NEED their man to be emotionally attached to them, so that they can be sure that he will not leave them (and remove his protection and support). Thus, women will OFTEN forgive a man for cheating, if it is just sex. As long as there is no risk of him withdrawing his support and emotional attachment, it is a forgviable sin. What is unforgivable to a women is when a man becomes emotionally attached to another women - because that creates a HUGE risk that he will withdraw his support and protection. (By the same token, it is generally unforgivable for a women to sleep with other men, from the man's perspective - because she could potentially become pregnant with another man's DNA, and he could be stuck supporting and portecting someone elses' DNA - as a cuckold).
If a WOMAN cheats on a man who is already emotionally attached to her, it means that she does NOT consider him an adequate mate, and is looking for others to replace him with. It is that simple. Women do not cheat just to get some sex or a thrill. (Unless they are severely fucked up and got their mental wires crossed - in which case they are not a good mate either!)
I'm not necessarily saying this is how people THINK today - but that doesn't matter, because that is how our brains are designed (from aeons of selective breeding and survival pressures)
So, the ONLY way that she isn;t going to do it again is for you to find out WHAT it is that you are not providing for her in the relationship, and find a way to provide it. If you cannot, stop wasting your time, because she WILL cheat again, and she will leave - as soon as she finds someone better than you. And she WILL be looking for that person.