The first Pārājika in the Bhikkhunī Pātimokkha reads:
Should any bhikkhuni willingly engage in the sexual act, even with a male animal, she is defeated and no longer in communion.
Now at first glance it would seem that a nun could break this rule.
But once a Pārājika rule is broken, a nun is no longer in communion; is no longer a nun and can then therefore do as she likes because she no longer falls under the Vinaya. So for instance, if a nun breaks Pārājika 2, the rule on not stealing, she is no longer a nun. So if she has sexual intercourse after that, it no longer matters, she is out of communion already.
The Bhikkhunī Pātimokkha has however 8 rules in total, while the Bhikkhus only have the first 4 of those rules.
Rule 5 reads:
Should any bhikkhuni, lusting, consent to a lusting man’s rubbing, rubbing up against, taking hold of, touching, or fondling (her) below the collar-bone and above the circle of the knees, she also is defeated and no longer in communion for being ‘one above the circle of the knees.’
And Rule 8:
Should any bhikkhuni, lusting, consent to a lusting man’s taking hold of her hand and touching the edge of her outer robe, and should she stand with him and converse with him and go to a rendezvous with him, and should she consent to his approaching her, and should she enter a hidden place with him, and should she dispose her body to him—(any of these) for the purpose of that unrighteous act—then she also is defeated and no longer in communion for ‘eight grounds.’
Now please tell me how any Bhikkhunī could possibly break Pārājika 1 without first breaking rule 5 or 8 or both (unless it is with an animal, but in most cases that would be very unlikely)?
Now Pārājika 1 is not actually mentioned at all in the Bhikkhunī Vibhaṅga. Only what we know as rules 5 to 8 are mentioned there. The Bhikkhunī Pātimokkha as a whole has been compiled at a later date and is part of the commentaries. Also the part of the Bhikkhunī Vibhaṅga that is actually in the Sutta Vibhaṅga of the Vinaya is very messy. There are rules from the Bhikkhu Pātimokkha and from the Khandhakas brought into it, probably after the Bhikkhu Pātimokkha was already closed. The Bhikkhu Pātimokkha itself is a very clear rounded structure, with parallels in all other schools; in fact, it is the same list of rules everywhere. The parallels and wording in the various schools is far less clear in case of the Bhikkhunī Pātimokkha.
Ayya Kathrin @vimalanyani is currently working to translate the Bhikkhunī Pātimokkha of the various Chinese schools and we will have a clearer picture after that, but a lot of work still needs to be done to further our understanding about how the Bhikkhunī Pātimokkha came to be.