There is an important relationship that would fit your question in Republican, Principate, and Imperial Rome - the cliens-patronus. The patronus held a higher social position, and was responsible for providing protections and influence on behalf of the cliens. The cliens was generally poorer than the patronus, although he could be rich in his own right. Generally, the cliens provided a service of some sort to the patronus, and together, they formed what could be considered a backbone relationship of Roman society.

The cliens would not be so able to switch allegiances, being the person of lesser social standing. There is evidence, however, of a patronus leaving the cliens with nothing (cf. Livy IX). In Livy, we find a cliens providing sexual services to his perverted patronus, who then leaves him with nothing when another, cuter man comes by.

Regarding your first question about taboo, I can't speak to any sanctions that would result in a cliens betraying his patronus, but I suspect that any patronus, using influence and political powers, could destroy the life of his cliens, if he so chose. The thing to keep in mind with the Satire by Livy that I mentioned earlier is that Livy's point is to show a certain degradation in Roman culture - the betrayal of the cliens by the patronus was considered a horrific act. Livy was not at all concerned about the sexual proclivities of the two involved, rather, the immorality of a patronus abandoning his cliens.

Read this Wikipedia article:

https://en.wikipedia.org/wiki/Patronage_in_ancient_Rome

It seems to be spot on, and has quite a few references to follow.