Am I condemned for an extramarital affair? A long time ago I had a romantic relationship with a married woman, and later I regretted. How is this seen in Buddhism?
It is classified as sexual misconduct. According to the Cunda Kammaraputa Sutta:


  And how is one made impure in three ways by bodily action? There is
  the case where a certain person takes life, is a hunter,
  bloody-handed, devoted to killing &amp; slaying, showing no mercy to
  living beings. He takes what is not given. He takes, in the manner of
  a thief, things in a village or a wilderness that belong to others and
  have not been given by them. He engages in sensual misconduct. He
  gets sexually involved with those who are protected by their mothers,
  their fathers, their brothers, their sisters, their relatives, or
  their Dhamma; those with husbands, those who entail punishments,
  or even those crowned with flowers by another man. This is how one is
  made impure in three ways by bodily action.
  
  http://www.accesstoinsight.org/tipitaka/an/an10/an10.176.than.html


However, that doesn't mean that you are "condemned." You did something that wasn't right, but if you recognize that you did something wrong and strive to do better in the future, that's all you should do. In the Samaññaphala Sutta King Ajatasattu confessed to the Buddha that he had murdered his own father, which in Buddhism is one of 5 Garu Kammas, the 5 most heinous acts a person could possibly commit:


  "...King Ajatasattu said to the Blessed One: "Magnificent, lord! Magnificent! Just as if he were to place upright what was overturned,
  to reveal what was hidden, to show the way to one who was lost, or to
  carry a lamp into the dark so that those with eyes could see forms, in
  the same way has the Blessed One — through many lines of reasoning —
  made the Dhamma clear. I go to the Blessed One for refuge, to the
  Dhamma, and to the community of monks. May the Blessed One remember me
  as a lay follower who has gone to him for refuge, from this day
  forward, for life.
  
  "A transgression has overcome me, lord, in that I was so foolish, so muddle-headed, and so unskilled as to kill my father — a righteous
  man, a righteous king — for the sake of sovereign rulership. May the
  Blessed One please accept this confession of my transgression as such,
  so that I may restrain myself in the future."
  
  "Yes, great king, a transgression overcame you in that you were so foolish, so muddle-headed, and so unskilled as to kill your father — a
  righteous man, a righteous king — for the sake of sovereign rulership.
  But because you see your transgression as such and make amends in
  accordance with the Dhamma, we accept your confession. For it is a
  cause of growth in the Dhamma &amp; Discipline of the noble ones when,
  seeing a transgression as such, one makes amends in accordance with
  the Dhamma and exercises restraint in the future."
       http://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html


The Buddha did not condemn King Ajatasattu for his evil act, but encouraged him to have a change of heart and live an ammended life. He did the same for Angulimala the Serial Killer (Angulimala, his nickname, means garland of fingers because he kept his victim's fingers on a string as a garland) having him become a Monk and when Angulimala saw a woman in childbirth and wanted to help, the Buddha said this to ease Angulimala's feelings of guilt over killing so many people:


  "In that case, Angulimala, go to that woman and on arrival say to her,
  'Sister, since I was born I do not recall intentionally killing a
  living being. Through this truth may there be wellbeing for you,
  wellbeing for your fetus.'"
  
  "But, lord, wouldn't that be a lie for me? For I have intentionally
  killed many living beings."
  
  "Then in that case, Angulimala, go to that woman and on arrival say to
  her, 'Sister, since I was born in the noble birth, I do not recall
  intentionally killing a living being. Through this truth may there be
  wellbeing for you, wellbeing for your fetus.'"
  
  http://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html


The Buddha taught people guilty of the most heinous things to change their ways but to move on and not dwell on their previous errors. If the Buddha encouraged a man guilty of killing his father and a serial killer not to dwell on guilt, how much more would he encourage someone who committed adultery to simply have a change of heart and not dwell on guilt.