Regulative principles for sex life
First of all, let me explicitly state that I assumed that you are asking about the rules & regulations that a husband and his wife should abide by.Because ,sexual activities, of any other forms are clearly stated ,in Shastras, as Adharmic.
Well,except for few prohibited days ,like the days of period,and few others,sexual activities are perfectly legal between a husband and a wife.
Here are some relevant rules from the Manu Smriti:
Manu 3.45. Let (the husband) approach his wife in due season, being constantly satisfied with her (alone); he may also, being intent on pleasing her, approach her with a desire for conjugal union (on any day) excepting the Parvans.
And, here are the restricted days:
3.46. Sixteen (days and) nights (in each month), including four days which differ from the rest and are censured by the virtuous, (are called) the natural season of women.
Manu 3.47. But among these the first four, the eleventh and the thirteenth are (declared to be) forbidden; the remaining nights are recommended.
A husband who avoids his wife on these 6 forbidden nights is as good as a celibate(a brahmachari).
Manu 3.50. He who avoids women on the six forbidden nights and on eight others, is (equal in chastity to) a student, in whichever order he may live.
The Parashara Smriti says that a husband should always approach his wife on the day her courses cease,otherwise he commits an offence.
If the husband meets not his wife on the day that her monthly courses cease, and she be near at hand, he incurs the awful sin of having killed a foetus. There is not the slightest doubt herein.(Parashara Smriti)
& similarly ,for the wife,it says:
If a woman meets not her lord, on the day that her monthly courses stop, — she after death goes to hell, — and becomes a widow in repeated births(Parashara Smriti).
And, I don't think there are any more rules other than those mentioned above.
EDIT-Also one more thing I forgot to add is that sexual activity is usually prohibited during daytime.
Here is a reference from the Apastamba Dharma Sutras book.which refers to Manu,but i'm yet to find where in Manu it is so stated.
In any case,i think,that sexual activities should only be performed in night is mentioned in some more scriptures.Just need to find those references.
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