[mmxiii]

The problem is that this is your subjective interpretation of what YOU think the consensus is. This is what you think other people think is wrong. Why should someone on this board believe that your conclusions about the consensus are accurate, reliable, or valid? No one is interested in sorting that out. In contrast, the best information to offer here is experience, AKA a data point. Fundamentally your input here is suboptimal.

It's also really clear that what the consensus may be here would vary a ton by the actual situation: 1. Was the original relationship happy? Would the consensus be against this if it were an abusive relationship? 2. Was it just a casual girlfriend, or were they engaged? 3. Was the OP intentionally trying to seduce the girl, or did it just happen that they recognized it was a better pairing?

The reality is the consensus WILL differ based on the situation. There are a ton of shades here, but you fixated on the idea of "betrayal", as if all things with this pattern were uniformly bad. That's clearly not true, and there is simply not enough data for you to overfit and then moralize.

[crimsonalucard]

This isn't a science experiment. People walk through life creating conclusions and moralizing based off of subjective intuition. There is not enough information here to formulate a scientific conclusion but there is enough information to form a reasonable one.

That being said... consensus will differ based on the situation, but based on the information given the consensus is roughly the same: Don't betray your best friend. There are very few contexts where having an affair with your best friends girlfriend will be justifiably moral. Let me address some of the examples you gave:

The nature and context of the relationship between the boyfriend and the girlfriend itself is irrelevant and NONE of the OPs business. Although an abusive relationship justifies intervention, an unhappy relationship DOES NOT JUSTIFY AN AFFAIR. Intuitively, it is also highly Unlikely that the relationship is abusive. Make no mistake, anytime you engage in an affair with your best friends girlfriend it is most likely an act of utter betrayal.

If the OP was not intentionally seducing the girl but developed feelings naturally, it is the OPs' own business. It's not his fault he developed those feelings but it will be his fault if he acts on those feelings. He is now torn between his attraction to this woman and his guilt for betraying his friend hence his decision to query people on hackernews. We all have dark desires, but the desire itself does not justify the action.

I fixate on the affair itself and how the act of carrying out said affair is an act of betrayal. The situation and context of the action is not uniformly immoral and I can definitely empathize with the OP. However, despite all of this... you are still an ass hole if you have an affair with your best friends girlfriend. Most people can agree with this, except you.

I can't speak for you but it may be possible that your empathy for his situation is clouding your judgement.

[mmxiii]

No, you certainly can't speak for me. I have no empathy or relevant experience regarding betrayal, I simply wanted to point out how your response was more irrational than rational.

You are right the majority consensus is betraying your buddy is bad. But do you see how without details, it's not possible to give actionable and helpful advice? You are parroting an abstraction that isn't necessarily impactful advice. Certainly most people already know this, and yet this sort of situation is not uncommon. Maybe this advice isn't producing meaningful results? The fact is you don't know, because you don't have the relevant experience.

That's the first part of your advice. The second part is about deciding company ownership, and that's even more irrational, it doesn't really follow or is related to this relationship.