They’re likely basing it on a verse (that I don’t have handy at the moment) but it says something to the effect that if a mans wife is raped by someone he has to pay a certain amount of silver to the husband in compensation. I couldn’t tell you if it was old testament or not (probably though, sounds very old testament). I think the way it’s phrased was something like “if a man lays with another man’s woman, he must pay him X whatever”.