Shulchan Aruch Even HaEzer:
“A person must stay far, far away from the women, and it is prohibited to signal with your hand, to hint with your eyes, to any prohibited women. It is furthermore prohibited to laugh together with them and to be frivolous in her presence, or to watch her beauty. Even to smell her perfume is forbidden….”
Reb Moshe, in Igros Moshe EH 4:60, paskened that it is assur to talk to the opposite gender in a social context, and assur to be friends with them, and even seeming platonic relationships between boys and girls are assur min hatorah.
In the Sefer HaChinuch, mitzvah number 188, it states there that it is assur for boys and girls to talk to each other.
The Shulchan Aruch (E.H. 22:8) and the Aruch Hashulchan (E.H. 22:6) present examples of a man and a woman who grew up together or are related – such as cousins. Libo Gas Bo also applies to a man and a woman who are very friendly with each other, such as those who work together in an office or if the man and woman dated. Rav Shlomo Zalman Auerbach (cited in Nishmas Avraham 3:94-95) and Dvar Halacha (7:17), state that Libo Gas Bo applies even if the man and woman have only had minimal interaction such as a woman who has visited a doctor a number of times or a man and woman who had some business dealings together.
It is an issur d’oraysah. Rav Moshe, in the Igros Moshe, states that Lo Sikrevu means you can’t talk to girls. Rav Moshe is quoting a statment of Chazal in Avos D’Reb Nosson and rulings of the Ran and others. Avos: “Al tarbeh sichah im haishah” – Do not talk more than necessary with women. The Mishnah continues: “Whoever does talk more than necessary with women causes bad for himself . . . and in the end will end up in Gehinnom!” Re derech chibah: The Halachah (Rambam) prohibits negiyah even in a non-affectionate manner. Casual negiyah is also prohibited — see the Steipler’s Letters quoting the Chazon Ish for details — even if it doesn’t “mean anything”. Even regarding a simple handshake between strangers, Rav Moshe writes that you should not rely on this as a heter.
Rav Moshe addressed the Derech Chibah issue in his Teshuva, saying that whenever boys and girls are friends, even innocently, that is considered Assur, like Derech Chibah. R. Moshe Feinstein repeatedly (Iggeros Moshe, Orach Chaim vol. 1 no. 113; Even Ha-Ezer vol. 1 no. 56) ruled that a man enjoys shaking a woman’s hand and it is therefore
prohibited. The Steipler writes in his letters in the name of the Chazon Ish that it is absolutely prohibited. Regarding a so-called heter that you will embarass her if you don’t shake hands, it’s simple logic: If someone asked you to call them on Shabbos and you telligng them sorry I cant do that would embarrass them for asking, would you be mechalel shabbos to prevent their embarrassment? Or if someone offered you non-kosher food and refusing would embarrass them would you eat treif?
The Gemora (Shabbos 152a) quotes a posuk: “Ki hu amar vayehi” – Hashem decreed, and so it was! “Rav Kahane said, this is a woman. She is just full of dirt and blood, yet everyone runs after her”. Meaning, there’s no logic behind it, Hashem said it should be that way, and it is. It’s a Gezeiras Hakasuv. This is probably why, when Chazal say the angels came down to this world demanding a chance to keep the Torah, the aveirah that defeated them was Arayos. Because even though the angels were physical then, they still retained their angelic intellects which told them philosophically that money honor and power are not worth pursuing in this world. And through their great understanding of the futility of these Tavvos they rejected them. But women don’t work that way. No matter how wise you are the Taavah is still there, so the Malachim were unable to undo their Taavah for Arayos. The only way – the one and only way – to resist women is self discipline. Period. You can’t use your THINKING to fight the Taavah here, you can only use your ACTIONS. You have to resist, discipline, and STAY AWAY. This is why restraint from Arayos is called Kedusha (holiness) as opposed to refraining from any other sin. Refraining from other sins could be holiness, but it could also be simple wisdom. It could be you’re a Sameach b’chlko. Even a non-religious guy would be happier if he was a Sameach B’chelko, so not running after money does not necessarily have to do with religion or holiness. As opposed to Arayos, where “EVERY PLACE you find restraint from Arayos, there you find holiness” (Rashi Kedoshim). Because there is only one possible reason for not doing Arayos — self-discipline, and that’s what we mean here by holiness. This is also why we find more Gedorim and Siyagim against Arayos than any other sin. This is why, even though Sefardim always rule like the Bais Yosef even when the majority of poskim are against him, when it comes to issues of Arayos, writes the Chikrei Lev (II:180b), they are permitted to be strict, even against the Bais Yosef, with support of a majority of poskim.