This is not as simple a question as it appears. Most third-party sexual relationships, no matter what they are called, are started with PHYSICAL PLEASURE as the goal. If there are material/significant issues in the EMOTIONAL relationship before you add the extra cock, that's usually a red flag for the couple and their relationship...the bull/bf/extra cock will likely just expedite the break. Honestly, if you have those kinds of problems already, you probably won't be looking for a cock to add in. But, you can go in feeling comfortable...and things can change. Twenty years ago, I (male half) was the long-term "bf" with a couple where the wife became more attached to me. Started fine but then got messed up when hubby tried to play King of the Castle. There was also jealousy present...which is a total no-no if you are sharing your wife. Hubby was already controlling, to a degree, and made the HUGE mistake of trying to limit her access to me...while he had a steady gf himself with NO limits. That's a recipe for disaster...and they eventually split. Word to the wise: what's good for the goose, is good for the gander.