He wanted to do her justice, but he didn’t want to be disrespectful. Not only would it fail to dispel the reputation some of his tenants had of him, but it might anger a woman who knew how to wield a sword with deadly effect.
This was smartly written, because it is vague enough that he could refer to either Tsuruko or Motoko... considering Tsuruko's comments to her, and a locked door, I can just picture the angst Motoko must be going through at the moment...
Some thoughts though...
As much as Tsuruko may be tempted, and her comments to Motoko aside, she really should not pose for a nude portrait, even for her bedroom and private viewing.
If her goal is to actually have that other guy *know* she has a commissioned painting... as unless she is willing to show him it (something I doubt even she would want to do for a nude of herself), it should not be done.
On the other hand, if the goal is to secretly and privately be able to smirk and show this guy up without him knowing, that should give her pretty much free reign...
One aspect that may or may not have occurred to her, but may to Keitaro... is this could be only the first of potentially *more* commissions. I can see it getting around that two houses have his paintings and the one upsmanship of the higher families might well allow for multiple commissions, even ones with specific strings attached to not sell them.
Such a event coming to be would actually in the end make that guy that *bought* his painting look less in the eyes of others that specifically sought him out and had them commissioned.
This type of setup would be difficult in the fact that Keitaro wants to remain hidden from public eye, but is *not* impossible, such families could use the Aoyama family as a o between and a promise to not directly seek out or divulge his identity.
This would be a nice financial windfall for Keitaro and a boon to the Aoyama both in status (business/social ties to keitaro) and personal ties to Keitaro, were the goal a Tsuruko match.
All that would be for long term story, and does not help much here.