Cheating and having affairs can break peoples's hearts and I typically do not approve of having an affair. It can poison a relationship and make love stop and be replaced by a burning hatred! What a terrible thing! Love is amazing! However, if you are in an arranged marriage with someone you don't love and you can't divorce or leave, then I think it is morally acceptable to have an affair with someone you love because your love for them is denied and there is no reason to deny your own happiness when your own rights are being violated. However, this case is very particular. It's better to break up or divorce first,, but sadly you can't always do that. Plus, an arranged marriage without love (some people find love) is not a marriage I would want anyone to be on.
Unfortunately it is often done, but having a commitment means having a responsibility for the person you are committed to. It is true, there are arranged marriages where people are forced together. However they often find love, tolerance, or both. Many also believe that it is for the best, that their parents made the best match possible for them. The point is, they make it work.
And in voluntary marriages, there are many options to having an affair and hurting who you once loved. An affair is never acceptable. You have a responsibility to the person you are married to. And having an affair is breaking both the responsibility and the commitment.
We live in a world where if something is broken, we simply throw it away. However a marriage is a special bond that should be fixed when broken. Not thrown away with an affair.
Being in a relationship means having a commitment. When you and your partner are committed to each other, it is never okay to have an affair. You are with your partner for a reason and if you feel that you do not have a strong bond with them, break up with them and move on. Just don't cheat.