When adultery is a factor in the decision to divorce, both parties wonder what impact, if any, the infidelity will have from a legal standpoint. The answer differs from state to state. In Illinois, the adultery is mostly relevant when it comes to "grounds", the legal reason that allows you to get a divorce in the first place.
Adultery can be used as a "ticket in" to the courthouse. It gives the Court the authority needed to grant a divorce. Even when adultery took place in a marriage, adultery is rarely used as the grounds as most people opt to claim "irreconcilable differences" to avoid having to talk about something emotionally painful. At the end of the day, as long as grounds are established, which grounds are used doesn't matter because under Illinois law, marital misconduct cannot be used as a basis for deciding how property & assets will be divided. That means that in deciding, for example, who gets the house or how much spousal maintenance is to be paid (if at all) - the court doesn't care if one party cheated on the other. It cannot be considered.
One caveat or exception comes about when a spouse spends marital money for non-marital purposes, for example- if husband spends money taking his girlfriend on a vacation or buys gifts for her. In that case, he has widdled away money that belongs to the marriage and may receive a smaller distribution to offset this "dissipation".
In terms of adultery and child custody, it may or may not matter. In other states, adultery is much more of a big deal when it comes to financial decisions made by the court in divorce cases.