I think that should be considered the third coworker's problem. Sex is a natural and normal bodily function. Imagine if someone was insisting that anyone menstruating stay home.
Sure, in the way that peeing all over the walls is a bodily function. It's a bodily function that's allowed at home (if you must), but not allowed in the office.
Sex is voluntary.
Not the same thing at all. Have sex somewhere that's not the office. It's honestly not hard.