I would like to know what is it true that the Theravada Buddhism promotes female inferiority?

The Thai Sangha seems to have a low opinion of women, but there are Theravada bhikkhunis. The Syamnikaya Theravada tradition in Sri Lanka allows full female ordination. Thai Theravada is not all of Theravada.

Santi Asoke, a Thai Theravada movement outside the mainstream Sangha has sikkhamats. These women are not fully ordained, but lead a much more monastic lifestyle than the mae chees, the white-robed 10-precept female renunciants found in the main Theravada tradition.

I'd say that the very fact that females can't just ordain like males clearly says that females are regarded as inferior. However, don't make that a problem. All females should be happy being born a such. They thereby has been given a whole lifespan where they can work on the bad karma that brought them their current miserable situation so they have the chance to be reborn as a real human next time around. Note: That was sarcasm and not my opinion. - I know you have to truthfully answer 'Yes' to the question of whether you are a whole (that is with intact appendices) human when ordaining, but is that a man or Buddha given requirement?  (Joking aside, I would appreciate a documented answer to this).

In Theravada one of the requirements to be a bodhisatta is to be male (Lopez, D. The Story of Buddhism, HarperOne, 2001, p.66). This is not the case in Mahayana/Vajrayana. Kuan-yin is female, as is Vasudhara, though these examples may perhaps be seen as either a feminine deity (Vasudhara/Lakshmi) or a manifestation of a previously male entity or principle (Kuan-yin/Avalokiteshvara).

Early Mahayana texts, I gather (http://www.answers.com/topic/bodhisattva), stipulated masculinity as a condition for bodhisattva status, but later texts allow for feminine bodhisattvas. Mahayana, unlike Theravada, is developmental, and later texts, where accepted into their relevant Canon (Tibetan, Chinese, etc.) take precedence over earlier ones where they are in conflict.

For more informations about the topic look this link. You can follow other informations on the base of this link.

http://www.congress-on-buddhist-women.org/index.php?id=59