Over the years there has been some purist anxiety about whether due to or owing to can properly predicate a cause of a clause (the way because can) as well as of a noun or its syntactic equivalent; but that is not really the issue here. The intent is clearly to identify a cause for the flexibility of the vagina, and not for the vagina itself, its existence, flexible or otherwise; but this last is what the syntax actually implies. Thus substituting because [of] for due to will not fix the problem. One could, however, write something like the following:
Aside from the flexibility of the vagina [noun phrase], which is due to the elasticity of the pelvic muscles, . . .
Aside from the effect on the vagina, which is/becomes flexible [clause] because of [or, yes, due to or owing to] the elasticity of the pelvic muscles, . . .