Walter Sinnott-Armstrong argues,
Let’s assume that God commanded us not to rape. Did God have any reason to command this? If not, his command was arbitrary, and then it can’t make anything morally wrong. On the other hand, if God did have a reason to command us not to rape, then that reason is what makes rape morally wrong. The command itself is superfluous. Either way, morality cannot depend on God’s commands.
The premises of the argument can be summarised as follows,
 Either, (i) there is a reason, r, why God prohibits rape; or, (ii) there is no reason, r, why God prohibits rape.
 If there is no reason, r, why God prohibits rape then Gods commands are arbitrary.
 If there is a reason, r, why God prohibits rape then, r, is what constitutes the wrongness of rape.
I think this argument is problematic; the problem is that the word “reason” is ambiguous. William Wrainwright notes that the word reason can be used in two different senses. The first is a constitutive sense; one affirms that the reason water has certain phenomenological properties is because it is H20. In this sense, the use of the word “reason” denotes a special kind of ontological relationship. The second sense is a motivational reason; as in, when I state that the reason I feed my daughter is because I love her. This sense is more psychological or epistemological.
It is important to note that these two senses are not the same as the following illustration demonstrates. Noah fills a glass with water. If we ask what the constitutive reason was for his action, the answer would be that he filled the glass with water because he filled the glass with H20. If we ask what the motivational reason was for his action, the answer would be that he wanted a drink. Yet, his wanting a drink does not constitute water, likewise water being H20 is not the motivational reason he wants the drink.
When Armstrong states, “Let’s assume that God commanded us not to rape. Did God have any reason to command this?” he could be asking if there is a motivating reason as to why God prohibits rape or he could be asking if there is a constitutive reason as to why God prohibits rape. Either way, however, his argument fails.
Turning to the first option, if Armstrong means to ask, did God have a motivating reason for prohibiting rape? then r refers to a motivating reason and premise  is correct. If God has no motivational reasons for prohibiting rape then God's commands are arbitrary. To avoid the conclusion that God’s commands are arbitrary one would have to concede that God has motivating reasons for issuing them.
The problem is that on this sense of “reason,” premise  is false. If r refers to a motivating reason then it does not follow that because r exists, r constitutes the wrongness of rape. I noted this in the example I gave above; the fact that Noah has a motivating reason to pour water into a glass does not mean that these motivations constitute him pouring water into the glass. What constitutes water are H20 molecules, not his motivations.
Armstrong could avoid this by denying that he means r to refer to a motivating reason, that he meant r to refer to some kind of constitutive reason. This might enable him to affirm that  is true. The problem is that if this is what is meant by r then  is false. Even if God does not have constitutive reasons for prohibiting rape, he could still have motivating reasons and if he does then  is false. If God has motivational reasons, such as concern for the welfare of others for issuing the commands he does, then God's commands are not arbitrary.
Armstrong’s argument therefore commits the fallacy of equivocation.
 Walter Sinnott-Armstrong “Why Traditional Theism Cannot Provide an Adequate Foundation for Morality” in Is Goodness without God Good Enough: A Debate on Faith, Secularism and Ethics eds Robert K Garcia and Nathan L King (Lanham: Rowman & Littlefield Publishers, 2008) 108.
 Ibid 106.
 Ibid 105; where he turns to the question of whether Theism is an adequate foundation for objective moral duties.
 William Wrainwright Religion and Morality (Aldershot: Ashgate Publishing, Ltd, 2005) 91.