the mc top and bottom kissed after the chapter 3 when they meet. the top should have broke up with his gf but nope he decided to said to his gf that he owes the esper bottom their lives because he save them, gf knows they probably kiss and more because he's now a guide and that's only way to help the estee. and top doesn't assured her he wouldn't do that. so you can say he cheated on her.
What happened on their first meeting is still debatable, for me. The kiss was very sudden that the guide even questions if it was real or not, there is no intention of cheating there. This is a guideverse universe so line is blur, especially if you have relationship with a guide, who is very much needed for the survival of the esper. If you see the esper, he is dying to the point that he thinks this is his last days and his wish was not to have sex with the guide but just to see the guide one last time as he closes his eyes. When you have live your entire life in pain and in the mercy of others and see a form of salvation, you would want to grab that too, but the esper did not want to 'own' or have the guide, he just wanted to experience that 'peace' one last time.
What the guide will do though from this moment, when he finally meet the esper is where we determine the 'cheating'. Though emotionally, there is already a start of it as the guide choose the feelings of the esper instead of his girlfriend.

A suicidal esper whose been too much pain and a guide tired of people around him who keeps demanding something from him. Interesting.. though I don't like if there will be cheating part, hope not.