That's an important point historically and doctrinally, Huseng. How many exceptions to that rule exist at present? There are many new and emerging forms of lay practice, but how many self-ordained practitioners are accepted as legitimate monastics within any Dharma community at present? Surely this is very, very rare. One is much more likely to encounter cases like this: http://dharmawheel.net/viewtopic.php?f=64&t=3949
According to the Brahma Net Sūtra 梵網経 one can, provided no suitable preceptor is available, do confession practices until signs appear, upon which one is a suitable vessel to receive the precepts, and therefore recites the precepts and thus receives them. The text states anyone, be they man, woman or slave, may receive the precepts, which generally include all the important rules that you would find in the vinaya such as celibacy and abstaining from violence, alcohol and music. Strictly speaking any valid preceptor is obligated to transmit the precepts if anyone comes asking, though in my experience so far I've never seen it actually work that.
So, technically if we consider the said sūtra as valid, we also can accept the existence of self-ordained monks, though in English we tend to call non-vinaya practitioners of this sort as "priests", though in East Asian languages this distinction is not made. A Theravada bhikkhu and a Japanese priest are both still called senglv
僧侶 in Chinese and Japanese respectively. However, in the west there is little knowledge of the vinaya, so the word "monk" is generally applied to anyone wearing robes and proclaiming themselves Buddhist.
There may be no physical preceptor behind such an ordination, but according to the text it is just as valid as if they did have a real person transmitting the precepts. Whether you accept their vision as a legitimate sign from the Buddhas of the ten directions that such a transmission took place or not is up to you. In history however it was generally not accepted because of the state and Buddhism being so closely tied. A self-ordained monk defied the rules of the state which sought to manage for themselves the sangha so as to prevent too many individuals from escaping mandatory civil service, which in ancient Japan and China would have been construction and agriculture.
I can see the value, especially having lived in Japan for three years, in a strict system of monastic rules, but then on the other hand it can lead to something akin to legal wrangling where there is all too much attachment to minor rules of the vinaya which only made sense in ancient India. Most of the rules were kind of "house rules" that were designed with young males between the ages of fifteen and thirty in mind. There were also a lot of crazies who ordained under the Buddha and as a result prohibitions on various activities were made only because of them. For example there is a prohibition in the Dharmagupta vinaya against bestiality because one crazy monk offered some food to a female monkey and then proceeded to rape her. As the vinaya explains this monk developed a relationship with the monkey where he would feed her in exchange for food. The other monks did not know if this violated the rules concerning sexual activity or not, so they went to the Buddha to ask. Of course he laid down the rule and said bestiality was forbidden.
This just goes to show you how some crazies signed up for the early sangha and as a result many many rules had to be laid down as a result.