The BKBh offers multiple reasons for this: people might become suspicious seeing a nun accepting clothes from a man; the nun might become greedy; someone might put a spell on her; being weak, she might not be able to carry the clothes; or nuns might end up quarreling over them among themselves (4153-58).
Assuming such a role is not a minor commitment: "If one ordains [women], he must protect them as long as he lives" (jai puna pavvavei, javaj-jivae tau palei | BKBh 1063).
He may have to give the nuns their requisites--since they cannot receive them themselves--or confirm a female novice, reassure nuns who are contemplating leaving the order, or console a nun whose relative has died (BKBh 3722-23).
If they are constantly participating in each others' affairs, conducting their rituals together, living in proximity, and even caring for each other at times of illness (e.g., BKBh 3769-803), there is a very real danger that they would be perceived--and possibly even come to think of themselves--as an alternative kind of family.
iya samdamsana-sambhasanehi bhinna-kaha-viraha-joehim | sejja-tarai-pasana, voccheda dudittha-dhamma tti || BKBh 3713 In this way, as they look at each other and talk to each other, with deviating conversation and private association, the host, etc., seeing it, discontinues [his support]: "They have understood religion poorly." He also warns that if a monk, afflicted by hunger, thirst, or heat, enters a temple or an empty house to rest, and a nun is seen entering the same building, people will suspect a rendezvous (BKBh 2181-91).
He acknowledges that, although nuns have given up laywomen's decorations, baths, and unguents, have covered up their bodies, and are practicing austerities, they still possess feminine beauty, evident in the shape of their bodies and smooth skin (BKBh 2101-2).
iya samdamsana-sambhasanehi samdivio mayana-vanhi | bambhai-guna-rayane, dahai an-icchassa vi pamaya || BKBh 2152 The fire of passion, lit by gazing and conversation, due to carelessness, burns the jewels of the virtues of celibacy.
Expiations are assigned for monks who approach the nuns' lodging without a good reason, and for nuns who allow them to enter (BKBh 3681-83).
tasim kakkh'-antara-gujjha-desa-kuca-udara-uru-m-aie | nigghahiya-indiyassa vi, datthum moho samujjalai || cinta ya datthum icchai, diham nisasai taha jaro daho | bhatta-a-royaga muccha, ummatto na yanai maranam || BKBh 2257-58 If one sees their [the nuns'] armpits, private parts, breasts, belly, or thighs, delusion can blaze up even in someone whose senses are subdued: thinking about her, wanting to see her, sighing, fever, burning, lack of appetite, infatuation, going crazy, becoming non-cognizant, dying.
For example, the BKBh discusses situations in which Jain nuns can stay with their own relatives (BKBh 2675, 3236) or be escorted or protected by them (BKBh 2070, 2445).
Likewise, the nuns will be coveted if in the refuge of a weak person, or without a refuge at all" (sunna pasu-samghaya, dubbala-gova ya kassa na vitakka | iya dubbala-nissa-nissiya va ajja vitakkao || BKBh 2437-40).
Abbreviations of primary texts ApDh Apastamba-dharmasutra BauDh Baudhayana-dharmasutra BKBh Brhatkalpabhasya BKT Brhatkalpa-tika GauDh Gautama-dharmasutra MBh Mahabharata MDh Manava-dharmasastra Nar Narada-smrti Par Parasara-smrti VaDh Vasistha-dharmasutra VavS Vyavahara-sutra VavBh Vyavahara-bhasya Visnu Visnu-smrti VS Vinaya-sutra VSS Vinayasutravrtiy-abhidhana-svavyakhyana = 'Dui ba 'i mdo'i 'grel pa mngon par brjod pa rang gi mam par bshad pa (Derge Vol.