I'm not sure your analogy applies here. Opioid substances aren't capable of producing persistent subjective improvement because the brain seems to have defense mechanisms against it that neuroscience has not yet found a way to bypass, but in the case of a sufficiently advanced AI companion, the brain would be receiving the exact same inputs as with a real partner.

With many people choosing not to reproduce, many currently existing relationships are already primarily a sort of recreational activity and are divorced from their evolutionary purpose. Replacing live partners with AIs would just take that one step further.