love

Pretty bad, methinks!

difficult question to answer

It depends on what you mean by 'relationship'.

It is the ultimate betrayal. You are betraying the trust, faith and love of your spouse. Once the trust is gone, the relationship, even if salvaged, will never be the same. For most people, it is the end of marriage because the loss of trust leads to suspicion (is he/she still cheating). There is an old saying, "once a cheater, always a cheater". That saying exists because most people who cheated, repeat the behavior.