It is reality of life and applicable to DMG relationships also. Marriage is the highest form of relationship (life long commitment which is also legal) between man and woman (where sexual relationship applies). Relationships are commitments. If you have a friend from opposite sex whom you date and its a secondary form of relationship (since you are not bound by law) but still its a commitment. You go out together and stick to each other and either marry or move forward. (You can split without going to the court). In both these relationships you don't cheat. That is you are not romantically involved with two persons concurrently. But if you don't click with each other you split and then you have a choice to move on.
What people call love is a love of longing (sexual attraction) for the other partner or WANT to be together. Meeting twice a week for 2 hours is not love, its more of infatuation. The real love aka caring and sharing with each other starts once you start living together as husband and wife. In sickness and in health. How much you care for each other and how much you give to each other 24/7, 365 days. This is why love marriages fail too miserably, there expectations of life is totally different. There is a big difference between Being Together and Living Together. Once you start living there would always be a difference of opinion but that does not effect love unless you have a strong ego and you want to rule the other person by force.
In DMG, Riddhima moved from secondary relationship of being Armaan's girlfriend/love and chose to higher form of commitment by marrying Sid. Since Armaan rejected her (or left her lying unconscious atleast and never asked her to wait for him) once she went after him, as she was not legally or morally bound by her commitment she opted for a higher commitment with Sid. Which may seem bad but that is what one normally does unless he is a DevDas.
Now once Riddhima is married to Sid (she made her choice) she should not get involved romantically with Armaan as this would be termed as cheating. If she accepts the reality of her commitment with Sid and sticks to it, then the way they started to think about each other giving themselves a chance to settle down, before Armaan stepped in, with time, real love can develop between Riddhima and Sid. If you want your wound to heal you stop scratching it every now and then. Coming across Armaan everyday would not help her heal and move on. Look around, how do couples start loving each other once there marriage is arranged by their parents. In my opinion everyone has a past but only those who want to be happy, live in the present.
Would you characterize Riddhima as a cheating wife?