The porn video tag "father-gay" refers to a specific category within the gay porn genre. In this case, it indicates that the content involves a father and another man engaging in sexual activities. This type of content can vary greatly depending on individual preferences, but typically showcases an older man (who is often portrayed as the father figure) with a younger partner, who may be a son's friend, neighbor, or any other relation to the father character. The main idea is that there is an age difference and the relationship dynamic adds an element of taboo or forbidden pleasure.