Many years ago there was a debate (written) between an ex-monk Sangharakshita and a current monk called Ajahn Brahm.
Sangharakshita was ordained as a Theravada monk in India. Unbeknown to him at the time many of the monks that presided in his ordination ceremony, in around 1950, were monks that had commited parajika offences and therefore were not monks. His upajjaya had a wife and child living with him at his temple etc. Sangharakhsita only came to know this many years later and wrote that he because of this his ordination was invalid and he had never actually been a monk. He then used this to justify his own engagement in sexual practices including with his male students.
Brahm wrote an article saying Sangharakshita was wrong and that he had been ordained, validly, despite one or 2 monks being invalid monks.
Sangharakshita then wrote another little booklet, "was the Buddha a Bhikkhu?", criticising this view and the general view of some Theravadins that they are too concerned with the minute details of obscure rules etc and miss the spirit of the teachings.
The great ex monk Laurence Mills (Khantipalo), a former student of Sangharakshita briefly in India, then wrote a little article in the Buddhist Studies Review about this little debate.
Most these materials are now online freely downloadable.
It is a fascinating topic and worth reading.
Keep in mind that Sangharakshita is critical of Theravadin hard heads and then concept that they are orthodox Buddhists.