There is an obvious comparison to porn here, even though you disclaim 'not catgirls'.

Anyhow I think the merit of such a thing depends on a) value calculus of optimization, and b) amount of time occupied.

a)

• Optimization should be for a healthy relationship, not for 'satisfaction' of either party (see CelestAI in Friendship is Optimal for an example of how not to do this)
• Optimization should also attempt to give you better actual family members, lovers, friends than you currently have (by improving your ability to relate to people sufficiently that you pass it on.)

b)

• Such a relationship should occupy the amount of time needed to help both parties mature, no less and no more. (This could be much easier to solve on the FAI side because a mental timeshare between relating to several people is quite possible.)

Providing that optimization is in the general directions shown above, this doesn't seem to be a significant X-risk. Otherwise it is.

This leaves aside the question of whether the FAI would find this an efficient use of their time (I'd argue that a superintelligent/augmented human with a firm belief in humanity and grasp of human values would appreciate the value of this, but am not so sure about a FAI, even a strongly friendly AI. It may be that there are higher level optimizations that can be performed to other systems that can get everyone interacting more healthily [for example, reducing income differential))