The huge poster hanging prominently at Berlin's Alexanderplatz square shows portraits of the three politicians next to the slogan, “What do these three men have in common? They should have used Ashley Madison.”
A spokesman for Seehofer, the state premier of Bavaria and head of the conservative Christian Social Union, refused to comment on the advertisement.
Seehofer had a four-year affair with 33-year-old Berliner Anette Fröhlich, who bore his child in June 2007. Seehofer then broke off the relationship and returned to his wife.
There could be legal consequences for the Ashley Madison agency for using the images of the philandering trio of politicians.
“The private lives of celebrities have no business in the public domain,” top media lawyer Ralf Höcker told the Süddeutsche Zeitung newspaper on Thursday. “You can't use them for advertising purposes without permission.”
Höcker has already had advertising campaigns banned for his celebrity clients, who have included model Heidi Klum and weatherman Jörg Kachelmann.
Höcker admitted that such adverts could be protected by freedom of speech laws if they use the celebrities' names or their public acts “ironically.” But he said simply using their private lives to draw attention to a product or service was not allowed.
Former Foreign Minister Joschka Fischer successfully sued Die Welt newspaper in 2006 by using a picture of his face altered to look like a baby for a campaign advertising its compact format. Fischer received €200,000 in damages from the paper.
Ashley Madison has over 10 million users, and claims to be the biggest agency of its kind worldwide.
Founder Noel Biderman told the Süddeutsche Zeitung, “Affairs are totally natural and sometimes even do a relationship good. But of course no-one should find out. Especially people in high office like Horst Seehofer should do everything they can not to get caught.”
Biderman added that the company had received no complaints about the poster so far.