While that's true, the Google targeting algorithm does an amazingly good job of selecting from my ad inventory.
Along with my art sites, in the same account, I have a couple of smaller sites devoted to wine. When I use Ad Review, I simply allow both types of ads to remain in the inventory and Google shows the wine ads on the wine sites and art ads on the art sites.
Since there are many more ads relating to art, there are times when art ads are shown on the wine sites but I've never seen wine ads on the art sites. Works well enough for me! YMMV
My primary concern if that the inventory is not polluted. The sites are designed such that targeted ads appear to be appropriate collateral material. If the ads are severely off base they look like... ads.