In Buddhism, the person whom we can easily associate with compassion is our mothers (besides our Gurus). So, when we contemplate on all sentient beings of having been our mothers before at one lifetime before (because our previous lives is as numerous as the sands on the riverbanks of the Ganges), we developed compassion for all beings. All mothers are female.
We, too were female once before in various lifetimes and so in Tantra, deriding women is deriding ourselves and our infinite potential for compassion that leads us to become fully enlightened. Hence, it is one of our sacred Tantric vows to always respect women.
Yes, during Nagarjuna's time practitioners were taught to meditate on the impurity of a woman's body so as to overcome desirous attachment ; especially so in the Theravaden tradition.The Theravaden school is likened to the roots of a tree, Mahayana the trunk and Vajrayana the branches and leaves. All three schools are to be learned and practiced as we progress.
With young children, parents teach them not to play with fire or electricity. When the children become older, parents will teach them the functions of fire and electricity, how to use them without getting hurt.
Similarly, as the practitioner progresses from the small scope to the medium and great scopes; to generate the Bodhi mind , he/she will contemplate the love of a mother which is the closest we can relate to in order to understand and develop in ourselves the unconditional love a buddha has for all sentient beings.
Just as it is said in the Lamrim, there is no contradiction in all of Buddha's 84,000 teachings if we know when to use each one according to our situations and needs.
If we understand this point, then being a woman is not an obstacle to our spiritual practice. I would see it as an aid because only mothers can relate easily in the contemplation of the sevenfold cause and effects to generate the mind of bodhichitta 
I love what both of you wrote, Big Uncle and Kurava.
Some Buddhists do tend to forget that we have had many different lifetimes as both men and women. And perhaps, even as animals or other beings. We become too fixated with our current form and fall prey to our attachment due to our current form. Hence, we do need to learn and understand the Dharma, step by step. Or scope by scope.
When the monks contemplate on the impurities of women, it is that women are impure or bad. It is the monks' ideas, desires or perception of women in general that may be impure. Hence, the monks need to contemplate and work on the very things that they are distracted by or are attached to.
Similarly, for all of us, if we are not ready or mature enough, or even stable enough to handle higher teachings, how can we expect to see results from it? Hence, we start from beginner's level and move on to the next. If we have not fully grasped the basic teachings and show some consistent results in practising these fundamentals, then it would be foolish to think that we can handle anything higher.
As children, we did not understand the dangers of some things - be it fire or electricity. Hence, as a result, we may get hurt. But once we learn and understand, we know how to best use these things such as fire and electricity to benefit us or people around us. It is no longer harmful or dangerous per say, but useful even.
Everything is not rigid and does not stay the same.
With higher understanding, we learn how to best utilize or even maximize something to bring about the greatest benefit to us and all around us.
It is when we do not understand something well or have wrong views of it, that's when something becomes dangerous or even fatal. And this is applicable to everything in our lives. Whether it is in our professional, personal or spiritual life.
The old saying, don't run before we can walk does have some wisdom in it.
