Jesus extended the definition of adultery to include sexual relations between a married man and a woman other than his wife (Mark 10:11-12, Luke 16:18). Other New Testament teachings also understand it that way (1 Corinthians 6:15-16, 1 Corinthians 7:2 ).
Therefore, for Christians, adultery is the sin of a married man having sexual relations with anyone other than his wife or a married woman having sexual relations with anyone other than her husband.
Christians believe in Jesus and his teachings. Jesus condemned adultery which is why Christians condemn it as well.