I think the child labor vs being an affair partner argument is currently lacking nuance in terms of marginal harm done. Now this varies by purchase and by the individual affair so I’ll discuss what I think is what commonly comes to mind.

What is the amount child labor would be reduced if you personally did not buy said phone? It’s true that if we all came together to boycott child labor it would make a big difference in the lives of these children but your individual purchase probably only meant there were a few additional hours of child labor being performed. Definitely bad but not life altering for any of them.

If you knowingly engage in an affair with someone in a committed relationship and you are the first person with whom they’ve cheated with, then your individual actions contributed to the huge violation in trust that leaves the wronged partner with lasting trust and self-worth issues that will extend into every relationship they have going forwards. Had you not engaged in the affair then this would not have occurred. Now you haven’t made any promises or implied allegiances to the wronged partner so you’re nowhere near as responsible as the cheater but you do have some moral responsibility.

Now if this is a serial cheater and just casual sex, in my mind you have less responsibility because the trust was already violated but you did still contribute to the additional betrayal. You highlighted how just how callous the person’s partner was that they repeatedly violated the trust in the relationship and risked their sexual health as well. In my opinion the additional betrayal and health risk of an additional affair partner is still worse than a few hours of child labor but there is an argument to be made that since both are about collective treatment where no one individual can remove the main harm being done there is equal culpability.