Adultery as one of the reasons for divorce in England is a well used ground for divorce because, like unreasonable behaviour, it offers quick divorce where it applies whereas the other three grounds for divorce involve compulsory long delays. Naturally, adultery is only relevant as a ground in UK divorce law where that has actually taken place. If there has been no adultery then the only other ground for "instant" divorce is unreasonable behaviour.
Incidentally, adultery involves sexual relations between one party to the marriage and an outside party of the opposite sex. If the sexual relationship is with a member of the same sex or if the relationship is not sexual that is not adultery. The appropriate ground on a divorce petition in such cases would be "unreasonable behaviour".
Another thing which is worth bearing in mind is that adultery is only available as a ground for divorce to the so-called "innocent" party. The person who has committed the adultery is not able to obtain a divorce on that ground unless his spouse is also guilty of adultery. This can sometimes cause significant problems. For instance, the person who has committed the adultery may well want to obtain a divorce and remarry but his/her spouse may refuse to grant a divorce on the grounds of the adultery. The usual answer to this is, "Well, you can either divorce me on the grounds of my adultery or I will divorce you on the grounds of your unreasonable behaviour." This may be problematic if there has been no unreasonable behaviour but the threat is often enough to resolve the impasse. In point of fact when a marriage breaks down there almost always is unreasonable behaviour.
It should also be remembered that there are time limits involved in adultery. A person who intends petitioning for divorce on this ground must do so within six months of becoming aware of the adultery (which is not the same as within six months of it taking place) otherwise he/she is taken to have "condoned" it. This is rather a quaint word but the reality is that many marriages can survive adultery and the law merely recognises the fact. However, this six months period only applies if the parties continue to live together after the discovery. If they have not lived together for more than six months after finding out about the adultery then this time limit is not relevant.
When people first become aware of adultery they are very often outraged and one of the first reactions may be to seek revenge. This is natural enough but one should be wary of the temptation to name the Co-Respondent (the outside party) in the divorce petition. This is because (a) it is not actually necessary to name the Co-Respondent and (b) not naming the Co-Respondent usually makes the divorce process easier. The reason for this latter is that if the Co-Respondent is named the divorce petition needs to be served on him/her. There can sometimes be difficulties of service when there is only one person to serve. To add to the number of people who must be served with the divorce petition merely doubles the chance of difficulty. The Petitioner should also bear in mind that the Co-Respondent has no particular incentive to co-operate by returning the papers to the court and this is exacerbated by the fact that that the person guilty of the adultery and the Co-Respondent may be ordered to pay the costs of the divorce by the court if there is no agreement to the contrary. Few people accept that cheerfully.
Strangely enough, there are often unexpected advantages in obtaining a divorce based on adultery. Such divorces are very often not "amicable" for obvious enough reasons. Nevertheless, what often happens is that the person who has committed the adultery either (a) feels guilty and/or (b) wants to remarry quickly. Each of these factors can very often lead to them agreeing to a financial settlement more unfavourable to them than they might otherwise have done. Although it may be scant consolation for a broken marriage the truth of the matter is still that either of the above two factors can lead to a more favourable financial settlement for the "innocent" party. If such is the case it is sensible to profit from the opportunity while it lasts because very often attitudes harden if the impulse is not acted upon quickly. This does occasionally present unexpected opportunities for resolving financial matters very quickly and favourably.
Incidentally, it is particularly important to be alive to this possibility because in UK divorce law the courts care not a jot that one of the parties to the marriage has committed adultery if they are asked to decide upon any matter relating either to the financial aspects or to the children. The fact of adultery is irrelevant to either of these legal decisions and the days of the courts "punishing" someone for adultery in questions involving child custody or capital division or whatever are long since over. One must therefore be particularly aware of the psychological rather than strictly legal advantages of this ground for divorce.