I think it is really important to point out a couple of things. First, according to Buddhism, the results of kamma are said to be complex; one does not simply experience a 1 to 1 result as, at least on the face of it, the above quotation of Newton's third law seems to imply. The Lonaphala Sutta, e.g., explains that whether or not one has developed her mind will have a very big impact on how the result of some unwholesome kamma will be experienced and that the belief that one must experience the result in equal proportion to the volitional deed is wrong view. For one who holds such a view, 'there is no living the holy life'.
Second, a kamma is not always productive of a specific rebirth, it can be the case that it produces its result within that 'life's continuance', meaning that it simply causes negative effects in this life. So, one committing sexual misconduct might only result in the 'trivial' result of 'rivalry and hatred' in this life. (Pa Auk Sayadaw, The Workings of Kamma)
Again, the way kamma actually produces results in an individual case is quite complex; in the Acintita Sutta the precise results of an kamma are said to be 'unconjecturable' and conjecturing about them to lead to 'madness and vexation.' If one reads the text cited above, The Workings of Kamma, one might get a sense for why this is said. According to that text and in line with the orthodox Theravadan view as I understand it, there are many kinds of kamma, wholesome and unwholesome, weighty and non-weighty, inferior and superior, there are frustrating kammas (which oppose kammas of the opposite kind, i.e., wholesome or unwholesome), there interceptive kammas, there are numerous mental factors that may make a kamma more or less unwholesome and so on.
Therefore, at last, it is not possible to give a straightforward answer as to what will happen to people who cheat on others in a relationship. We can be sure, however, that if mental states of greed, hatred, and ignorance were active, Buddhism holds the kamma was unwholesome.