Last week, Jekyll poked Lanyon's insecurities with a stick, so this week Lanyon stabbed him with a big, biphobic dagger.
There are so many ways Jekyll could tear down Lanyon's argument here (i.e. "You are also capable of being attracted to people other than me?" "Sir, we are literally not even in a committed relationship?"). But the whole point of mean, lazy comments like this are to induce shame, and shame has a funny way of shutting your brain down. And even if it didn't, neither Lanyon nor Jekyll have to tools to deconstruct the problematic beliefs they've absorbed from the world around them.