Women serving men food and drinks-Being served by a waitress:[1]

A woman may not serve a man an alcoholic beverage [i.e. Mezigas Hakos, although she may serve him other foods and beverages].[2] Some Poskim[3] rule that even other foods may not be served, just like is the law by one whose wife is a Nida. Practically, however, the widespread custom is to be lenient in these matters, due to several reasons:[4] Some[5] are lenient in this as they say the above action of affection is only forbidden to be done by one’s wife who is a Nida, while by another woman it is permitted to do so. Other Poskim[6] rule it is only forbidden to do it in private, while in a public area it is permitted. Other Poskim[7] rule that so long as one is not intending to have these actions done as a form of affection and rather his intent is Lesheim Shamayim, then it is permitted to do so. [Nonetheless, it is proper for one to avoid having a female waitress. If necessary, one should only do so in public, and should try to close his eyes, or look away, when being served, and she should do so with a Shinuiy, such as with her left hand.[8] It is certainly proper to avoid being served wine and other alcoholic beverages by a woman.[9]]

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[1] Even Haezer 21:5

[2] Michaber 21:5; Tur 21:5; Rambam Issurei Biyah 21:5; Kiddushin 70a “Shmuel says, one may not use a woman at all, whether young or old” and 81a; Yerushalmi Kesubos 5:6

The reason: As this can lead one to have immoral thoughts. [Tur ibid; Rambam ibid; Aruch Hashulchan 21:7] Alternatively, the reason is because one should not accustom a woman to be comfortable around men [as doing so can lead to sin]. [Rashi Kiddushin ibid; Aruch Hashulchan ibid]

[3] See Rama ibid in name of Binyamin Zev regarding eating from the same plate, and see Beis Shmuel 21:9 who extends this also to passing things to each other, and hence it seems that this opinion holds that all actions that are forbidden for a Nida to do for her husband are also forbidden for a woman to do for a man. So is also implied from Ezer Mikodesh 21:5 who includes serving food and drinks [and not just alcoholic beverages] as part of the original Issur.

[4] See Beis Shmuel 21:9 [in next footnote] that the leniency of the Rama is not with regards to Mezigas Hakos, however see Ezer Mikodesh 21:5 that states the custom is to be lenient even regarding using a woman as a waitress and they rely on the other Heterim listed here, such as that it does not apply when one is in public. So also rules the Aruch Hashulchan 21:7 that when the woman is offering the food in a way of Sheirus, it is permitted to do even Mezigas Hakos.

[5] 1^st opinion in Rama ibid; Rashba 1141

Other opinions: Some Poskim rule that the above leniency of the Rashba and Rama ibid is not going on Mezigas Hakos but on other actions, such as eating from the same plate or passing things to each other. [Beis Shmuel 21:9]

[6] 2^nd opinion in Rama ibid; Mordechai Kesubos 182

[7] 3^rd opinion in Rama ibid; Tosafus Kiddushin 82a

[8] Ezer Mikodesh ibid

[9] As according to the Beis Shmuel ibid the Rama’s Heter does not apply in such a case.