halacha - Tznius issues today as practiced at social events

What is the halachic basis for:

1. separate seating at simcah events i.e. weddings, bar/bat mitzvahs, etc?


2. members of the opposite sex not being allowed to commingle when there is no yichud?

Is this halachah or is this minhag, and what is the basis for a trend of more stringency?

Answer

Regarding separate seating at Simcha events i.e. weddings, bar/bat mitzvahs, etc.:

The Kitzur Shulchan Aruch in Siman 149:1 - The laws of Birkat Hamazon at Weddings - says:

Care should be taken that men and women do not eat in the same room. For if men and women eat in the same room then one does not say SehHaSimcha Bim'ono, as there is no Simcha when the Yezer Hara reigns.

Regarding members of the opposite sex not being allowed to co-mingle when there is no yichud:

The Kitzur Shulchan Aruch in 152:8 - The Prohibition of Yichud and other dealings with women - says:

A person has to keep his distance from women a lot - מאד מאד.

He goes on to forbid any casual interaction with women.

It's forbidden to wink with his hand or foot, or with his eye at a woman. It's forbidden to jest with her, or gaze at her beauty.

It's forbidden to smell a woman's perfumes, particularly if it's on her hand, or it's hanging on her body. It's forbidden to look at the colored clothes of a woman that he knows, even if the clothes aren't on her, lest he starts thinking about her.

If he meets a woman in the market, it's forbidden to walk behind her; rather he should hurry so that she is next to him or behind him. He shouldn't walk past the door of a woman of ill-repute, even if he's further than four Amot away.

The one who looks at even the small finger of a woman with the intention of getting pleasure from her, has sinned greatly.

It's forbidden to listen to a woman's voice, or to look at her hair.

The Kitzur (152:9) then goes on to discuss sending regards to a women:

One shouldn't ask how a woman is at all.

Even via her husband it's forbidden to send her greetings. Therefore, when writing to one's friend, it's forbidden to write Regards to your wife.

However, one is allowed to ask her husband or other people, how she's feeling. Similarly, one is allowed to write to one's friend: let me know how your wife is feeling.

Most of the above are already mentioned in various Gemarot, so they are not modern trends.