Sorry, but I'm not really a fan of your solution. You are limiting access to a focus based on 2 traits, one of which can only be gained through the same focus, while the other can also be gained by education. And you are limiting this to both AI and player, right? So, unless I get lustful through education I can forget about using this focus myself. But the problem is not what the player can do with the focus, it's (and always has been) the stupid and pointless shit the AI does with it. And in a large enough realm there will still be a couple of lustful assholes that will pick it at any time and go on a rampage. All this does is make it somewhat rarer, but I think a better way to do the same would be just reducing the base chance of AI picking seduction (which is what I'm going to do the next time I play it).
Of course, ideally the entire current system should be scraped and seduction made a plot that can be discovered by the ruler of the court of the target. Only without people joining the plot - plot power for seduction should depend on age, prestige, traits and most importantly rank of seducer and target - a king seducing a counts daughter should be relatively easy, a count seducing a princess much, much harder. And if the plot is discovered a wide range of options is possible, depending on how powerful the seducer is compared to the ruler of the court, and if the ruler actually cares about the target of seduction at all: ignoring it, asking him to stop, blackmailing him, exposing him, asking a common liege to order him to stop, setting up a trap, just imprisoning him, sending the target in hiding. And most importantly make the AI understand these rules and not commit suicide by them. Seducing a lowborn courtier in your own court or even a court of a vassal should be relatively safe and this is what the AI should spend his time on. Going after someone in the family of a neighbour should be much rarer, and actually going after someone higher rarer still. AI should take seduction, bang a few lowborns, produce a few bastards, take a lover or two for later and then switch out. That's it, there shouldn't be a bloody line in front of the queens bedroom.