Maggie Gallagher, blogging at National Review Online’s The Corner, suggests (more or less) a revival of the tort of alienation of affections, which has been abolished in the vast majority of states:
An updated tort of adultery could look something like the document below the fold. This tort (drafted for Minnesota, don’t ask me why) could be either expanded to include “commerical enterprises that intentionally and explicitly attempt to profit from acts of adultery,” or we could choose to limit it to commerical enterprises. Such a tort would not prevent websites from facilitating hookups that include adulterous ones. It would prevent them from explicitly incorporating adultery into their advertising plan.