Again, what is objectively good is specific to the being. This is where 'natural evil' (I personally believe calling it evil is misleading but that's what it's called) comes from, the differences in the objective goods of different beings.Something that is morally objective means that something is right or wrong in all cases, regardless of outside influences.
Your natural law asserts that the natural goal of sex is to procreate, and that because it is natural, it is good. Your natural law also asserts that these goods are objective.
The presence of an exception to this directly contradicts that natural goal and the idea of its objectivity.
Also, I think that reference is abit misleading in terms of God's relation to morality.
God, being self-necessary, doesn't adhere to a prior concept of perfection, so He isn't obliged to communicate any particular moral. God is being, He is perfection itself, meaning His act doesn't adhere to perfection, His act defines perfection.