How is sexual misconduct explained in regards to the five precepts? The 3rd precept is: 


  I undertake the training rule to avoid sexual misconduct.


How is this precept explained? 

Marriage is defined differently in different cultures, so are the commonly accepted rules in a given society considered as non-misconduct? If not, how should one determine the scope of misconduct according to the precept?
In MN 41 (and elsewhere), the Buddha describes sexual misconduct as follows:

He is given over to misconduct in sexual desires: he has intercourse with such (women) as are protected by the mother, father, (mother and father), brother, sister, relatives, as have a husband, as entail a penalty, and also with those that are garlanded in token of betrothal.
(Ñanamoli, Trans)

The commentary explains this passage as follows:

Māturakkhitātiādīsu yaṃ pitari naṭṭhe vā mate vā ghāsacchādanādīhi paṭijaggamānā, vayapattaṃ kulaghare dassāmīti mātā rakkhati, ayaṃ māturakkhitā nāma. Etenupāyena piturakkhitādayopi veditabbā. Sabhāgakulāni pana kucchigatesupi gabbhesu katikaṃ karonti – ‘‘sace mayhaṃ putto hoti, tuyhaṃ dhītā, aññattha gantuṃ na labhissati, mayhaṃ puttasseva hotū’’ti. Evaṃ gabbhepi pariggahitā sassāmikā nāma. ‘‘Yo itthannāmaṃ itthiṃ gacchati, tassa ettako daṇḍo’’ti evaṃ gāmaṃ vā gehaṃ vā vīthiṃ vā uddissa ṭhapitadaṇḍā, pana saparidaṇḍā nāma. Antamaso mālāguṇaparikkhittāpīti yā sabbantimena paricchedena, ‘‘esā me bhariyā bhavissatī’’ti saññāya tassā upari kenaci mālāguṇaṃ khipantena mālāguṇamattenāpi parikkhittā hoti.

Rough translation:

In the case where the father is gone, or the mother provides support such as food and clothing, the mother guards [her] thinking &quot;I will give [her] to a good family when she comes of age.&quot; This is called &quot;protected by the mother&quot;. By the same means &quot;protected by the father&quot;, etc., should be understood.
When families come together and, even when the fetuses have just reached the womb, make an agreement - &quot;If I have a son and you a daughter, let there be no getting to go to another; let her be for my son.&quot; Even in the case of a fetus that is protected thus, it is called &quot;having a husband&quot;.
&quot;Whoever goes to a woman of such and such a name, such a penalty is for him.&quot; Thus, a penalty is established by a village or a house, or a street; this is called &quot;as entail a penalty.&quot;
&quot;Even to the extent of those who are garlanded&quot; - to the last extreme of all, with the perception that &quot;This woman will be my wife&quot;, by tossing some garland over her head, there is &quot;those that are garlanded&quot;.

Soooo, it doesn't seem all that culturally dependent. Some points of note are:

Polygamy isn't included in the precept at all;
State-condoned prostitution doesn't seem to be considered immoral either. Illegal prostitution is considered wrong, but only because of the penalty it brings.
Infidelity doesn't appear either; in the Sigalovada Sutta however, the Buddha says that a wife is to be attended to &quot;by being faithful to her&quot;. It seems likely, therefore, that the reverse of the description here is to be considered as implied; if you are in any one of the protected situations yourself, engaging in a breach, even with a woman who is not so guarded, would be immoral.

Basically, the point is that breaching an existing relationship, including parental guardianship, is a breach of the precept. It doesn't seem likely that this would be affected by cultural norms; as the Buddha notes, as long as there is a claim and it is breached, it is wrong sexuality.
It's worth noting, of course, that in (Theravada) Buddhism all intentional sexual activity is inherently immoral; this precept just attempts to lay down some general principles for those unable to commit to celibacy.