I've got trouble understanding a line from a British rom-com film titled A Guide to Second Date Sex.
Sorry, a minor spoiler alert, but there is a scene where Laura, the female protagonist tells about her past relationship to Ryan, the male protagonist. She explains that she and her ex-fiance had to break up because it turned out he was gay and Ryan asks whether she saw that coming. And this is the dialogue afterwards:
Laura: Well, looking back now I can kind of see the signs but...
Ryan: Did his boyfriend give him away?
Laura: No, that... That was such a dad joke.
What I can't understand is why Ryan's words were a dad joke. What does 'give someone away' here mean? When I looked up the idiom, all I could find was 'to reveal someone's identity' or 'to hand over a bride to a groom', which I didn't think relevant to this situation. So I'm looking for your help.
Thanks so much in advance!:)