His "argument" against a rule that mandated rape must be read to be believed. I hasten to add that there is nothing to be said in favor of rape: to the contrary, it is obviously morally horrendous. But even here, Molyneux founders. He says,

If "rape" is a moral good, then "not raping" must be a moral evil — thus it is impossible for two men in the same room to both be moral at the same time, since only one of them can be a rapist at any given moment — and he can only be a rapist if the other man becomes his victim. (p. 66)