I believe Lamar probably cheated because all the answers he has given were vague and he seemed to act shifty all along. Khloe always seemed to have suspicions and I'm sure they had anchors in truth. And when that happens, it doesn't take long for the other party to notice and the marriage slowly breaks up...
In July 2014, Khloe Kardashian confirmed rumors that husband Lamar Odom cheated on her while they were still together. On her show, Khloe does not hold back as she expresses her disgust that he was with another woman and nowhere to be found on her birthday, forcing her to lie to her family and friends to cover for him while she hid out alone.
One can never know the inner goings-on of a romantic relationship, unless it is expressly stated. While I can't make a definitive argument for either side, I am inclined to say Lamar Odom did not cheat on Khloe Kardashian, mainly because there is no proof. Do I believe he was overwhelmed in a marriage to one of the most famous (justifiably or not) women in the world? Yes. Do I believe this could have led to infidelity? Of course. However, we will probably never know.
A lot of debate has been following Lamar Odom as of late- cheating, drugs, and all of it while keeping up with the Kardashian's. Rumors have been spreading about one, two, even three women confessing to sleeping with Lamar while he was with Khloe Kardashian. However, in all the "news sources" providing this information, none provide any credible evidence including texts, accurate witness reports, or confessions from Lamar Odom himself. It is all hearsay and thus, inconclusive if Lamar Odom cheated on Khloe Kardashian.