"No Fault" divorce" which is the law in Colorado, and in almost every other state, does not mean that you cannot get a divorce even if your spouse is at fault for having an affair, because fault doesn't matter. Instead, "no fault divorce" means that everyone who wants a divorce may, in most states (including Colorado) without the assent of their spouse, end the marriage after going through the necessary waiting periods and court hearings. The courts don't care why you want a divorce in a no fault case. Except for a handful of slight procedural nuances related to calculating deadlines and setting notices for court hearings, the courts don't care who actually filed for divorce in a no fault state. It is a relatively recent phenomena:
Oklahoma was the first state to introduce no-fault divorce in 1953, but most states followed suit only after then-Governor Ronald Reagan signed it into law in California in 1970.
Pennsylvania, which went to no fault divorce in 1980, was one of the last to do so.
There are a handful of states, most prominently including New York State (the others are Arkansas, Maryland, New Jersey, North Carolina, South Carolina, Vermont and Virginia), at least as of the last time I looked into the subject, that allow a couple to divorce without proof of fault only by mutual agreement. Unilateral divorces in those states are still permitted only upon a showing that the other spouse did something wrong and that fault wasn't mutual, although a divorce is sometimes possible without fault after an agreed seperation period and fewer and fewer spouses oppose a fault based divorce. I had to memorize all the grounds for divorce in New York when I took the bar exam. (Update: Gads, it is worse than I recalled, although a change in New York law is being discussed).
The idea behind no fault divorce is that we shouldn't force people who subjectively hate each other (even if the feeling isn't mutual) to have to concoct some reason to justify ending the marriage, and that we shouldn't force judges to hear people vent at each other over what happened in the past, when the marriage is going to end anyway. Before no fault divorce was invented, staged assignations observed by private investigators and divorces brought on the grounds of "emotional cruelty" were the norm. Before that (until the late 1800s or early 1900s depending on the state), only the legislature could give you a divorce.
In a no fault divorce state, everyone immediately assumes that the marriage will end, and we procede immediately to dividing property and debts, awarding mainenance (fka alimony) and child support, and allocating parental responsibilities. This is done first, on a temporary basis for the pendency of the divorce action, and then, "permanently" at the end of the divorce action (although child support and parental responsibility decrees are freuqently modified from time to time). But, if one of the parties dies before the divorce decree is entered (or at least, before a permanent financial division takes place) the parties die married.
The basic rule governing property division in Colorado, is that marital property and debts are divided roughly equally, with each party retaining their seperate property and debts (those that precede the marriage or are acquired by gift or inheritance, and have not been comingled, excluding any appreciation in those assets during the course of the marriage). Most states have roughly similar rules although the details vary.
The basic rule governing maintenance awards in Colorado is that they are in such amounts and for such duration as the judge thinks is fair, with some reference made to the duration of the marriage and the relative economic circumstances of the parties upon divorce. This is a virtually universal rule nationally. It is quite hard to modify a maintenance award after the initial award is made by a trial court.
The basic rule governing child support (which is basically uniform nationwide pursuant to federal law) is calculated based in the income of each divorcing spouse and the number of children involved, using a table, with certain exceptions that apply for very high income families and families with extraordinary expenses.
Parental reponsibilities (fka custody rights) are allocated between the parents (and occassionally others involved in the children's lives) in the manner the judge thinks is fair, which will be upheld on appeal unless the judge makes a screw up in how it is justified or grossly favors one parent or another without a good reason. The formal legal standard applied is the "best interests of the child". This is a virtually univeral rule nationally, although there are some differences on how some stituations, like a major interstate move, are handled from state to state.
Federal law greatly limits the ability of parents to venue shop in interstate child custody cases, by providing a fairly precise definition of a child's "home state" that must be the venue for custody actions related to that child except in rare cases.
Almost no state makes any serious distinctions in custody and child support cases between divorcing and always unmarried parents, except when it comes to establishing paternity.