A court may grant a divorce if your spouse has behaved so badly that you cannot reasonably be expected to live with them any longer. If you are seeking to divorce using the fact of unreasonable behaviour as grounds you need to make a strong case that your spouse has behaved in a way that you can no longer reasonably be expected to live with them.
Unreasonable behaviour is the most common grounds for divorce in
England and Wales, and can include bad behaviour such as physical
violence, alcoholism, abuse and mental cruelty, however it is also possible
to seek a divorce based on more mild grounds such as if your spouse cares
more about their job than their marriage, they don't spend any time with you
any more or you have no common interests.
If a sexual indiscretion other than sexual intercourse occurred with someone outside of the marriage, the grounds for divorce of unreasonable behaviour are often more applicable than that of adultery.
If however adultery did take place, you should instead file for divorce using adultery as the fact rather than unreasonable behaviour, though if your spouse
is not prepared to admit to the adultery publicly
and you are still on speaking terms it may be simpler to make a petition for divorce based on milder set of allegations.
This may be satisfactory under the fact of unreasonable behaviour and allow you to end your marriage
without making the details of any adultery public.
If one of you does not agree to the divorce, evidence and details of the unreasonable behaviour will be
needed. This evidence can come from witnesses such as friends or medical evidence, as well as your own personal account.