to reiterate in a logical way for you, the bolded you pointed out is logical unto itself;
b follows from a.
However, a is an assumption (logical term, look it up)
and a is unproven, therefore because god is unproven (logically), b is not proven by a, but may be proved by something else hypothetically.
As for appeal to nature:
procreation is part of our essential function, god wouldn't make gays.
procreation as a function=a
god doesn't make people gay=b
b follows from a, but a is an assumption and unproven as being essential. Therefore b is unproven by a, but may be proven otherwise, hypothetically.
As for strawman;
the argument implies that procreation is something that defines us. If we were defined by procreation, then the reasoning would be true. However, it is unproven that we are defined as such.
