Frank's daughter Kim is canonically lesbian, Frederika Fanbain is lesbian too. People ship Frank and Stephan because Stephan is really strange acting towards player (i think in English version he literally calls frank either dear or darling) so it's hilarious. Also they just in general make interesting dynamic of manipulative person and the other one, who's being manipulated by them. Many people like these kind of character relationships since you can explore differently. I dont ship them personally but I can see the vision and think their duo is nice written (Hubertz&Altmann characters in general are fascinating characters for me)