Too Many Explanations
Committed relationships are generally discussed before they are entered so both partners know what is expected of them. When they pair have progressed to the point where they feel comfortable with each other, suspicion is seldom part of their thinking. Players may never have thought they would be the ones to cheat on a spouse, but it can happen unexpectedly. Those who have made it a lifetime practice know how to avoid suspicion, but those new to the game will often offer too many explanations. This alone can make their spouse suspicious.
Time is what is mostly spent when a person is playing around, and accounting for it can be difficult. The person new to being a player will often feel the need to justify their time away from home. They may offer a timetable of their day when not asked, and that can begin the process of discovery for their significant other.
There are people who have always been players, and they feel little guilt or remorse about their behaviour. When a person is not normally that way, their lack of experience can be seen by someone close to them. They could feel guilty about their behaviour, or they could feel remorse for cheating. That might sound nice, but it is also one more indicator of their bad behaviour. Presenting a spouse with unexpected gifts can be the result, and that is what generally triggers suspicion when it occurs the first time.
There is no excuse for a person in a committed relationship to cheat on their partner, and those who are willing to forgive are presenting the player with a gift. It is easy enough to even break the legal bonds of matrimony, so cheating is about getting away with bad behaviour. While it may be something that occurs only once in a relationship, even that is far too much.