i know that after sexual intercourse it is necessary to wash full body, but does that always include washing hair as well? can we sometimes avoid making our hair wet and only wash the body?
The process of taking a bath after sex, or any janabah, includes washing of the head, but it's not necessary to wash all of the hair as this doesn't come in any teachings of the Prophet.
There are several hadiths on this issue. One from Umm Salma saying:
"O Allah's Messenger! I am a woman with braids on my head. Should I untie them for bath of janabah?"
The Prophet replied "لاَ إِنَّمَا يَكْفِيكِ أَنْ تَحْثِي عَلَى رَأْسِكِ ثَلاَثَ حَثَيَاتٍ ثُمَّ تُفِيضِينَ عَلَيْكِ الْمَاءَ فَتَطْهُرِينَ No, it is enough for you to throw three handfuls of water on your head and then pour water over yourself, and you shall be purified." [Sahih Muslim]
What does throwing three handfuls of water on your head mean?
It means that the Prophet would wet his head to the root of his hair. This is described by another hadith in Sahih Muslim that Rasolullah =
"would take the water and put his fingers in his hair until he reached the roots, and when he saw that the water had reached the roots, he would pour water over his head three times."
So, this is the proper way, as long as the water reaches the roots of the hair on your head, it's considered as bathing.
After sexual intercourse, you are in state of Janabah, and you have to perform Ghusl. your Ghusl is Not complete, unless every single inch of your body is not cleaned properly (so obviously you should clean hair properly (make them wet))
Here is scholarly opinion about this issue: http://islamqa.info/en/82344
And Allah knows the best