Hebrews 13:4
"Marriage [is] honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge."

This verse does not define adulterer. One must already know the definition of adultery from other verses or simply from Strong's Concordance. Since there was no Bible except the Old Testament at the time of the writing of the New Testament then every definition of adultery and fornication must be taken from the Old Testament; the only scripture ever quoted by the writers of the New Testament. Furthermore, the New Testament does not anywhere define adultery or fornication. Those terms are defined in the Old Testament and my articles site those definitions concisely.

A whoremonger is a pimp (monger = seller)(whore seller = bugaw in Tagalog).
whore - a woman who has had sex prior to betrothal/marriage See: What Is Fornication?
Note:
Engagement does NOT equal betrothal.
Consummated betrothal equals marriage.
Consummated engagement equals whoredom.

The difference between betrothal and engagement is that in most jurisdictions, engagement amounts to nothing more than a unenforceable promise. Furthermore, there is nothing in the law or custom of engagement that makes a man and his fiance become married simply by having sex. The case with betrothal is far different. Betrothal is a binding agreement which states that consummation shall result in marriage. There is a distinct difference between an unattached woman and a woman who is betrothed. The Bible gives the death penalty for a betrothed woman who lies with any man other than the man she is betrothed to marry.[1] There is no such distinction between an unattached woman and an engaged woman. Thus, engagement is quite worthless and without meaning. For most of today's women engagement amounts to an informal license to fornicate which they use to excuse shacking up with their fiance.

Hebrews 13:4 (above) states that marriage is honorable in all. This means that if a man is married then it is honorable. Was David married to his many wives? Yes. So it was honorable.Genesis 2:18, 22-25

Genesis 2:18 And the LORD God said, [It is] not good that the man should be alone; I will make him an help meet for him.

Gen 2:22-25 And the rib, which the LORD God had taken from man, made he a woman, and brought her unto the man. And Adam said, This [is] now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man. Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be o­ne flesh. And they were both naked, the man and his wife, and were not ashamed.These verses don't limit a man to o­ne woman unless a man and woman are also limited to just the amount and type of clothing that God made for Adam and Eve; coats of skins, not any material made from anything else.

Genesis 3:21 Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.If we follow the logic that we are limited to the number of things (in this case wives) that God originally gave to us then we are not supposed to wear any clothing other than animal skins because God didn't give Adam and Eve any other types of clothing. Of course this is ridiculous. You can have as man pieces of clothing and as many wives, as you can maintain comfortably.

Those who accuse men that have more than one marriage at the same time of adultery by twisting Genesis 2:18,22&25 are with or without knowing it, accusing righteous Abraham, Israel, David, and so many more great men in the Bible of knowingly committing adultery. In addition, they slander all Jews because if indeed the great patriarchs in the Bible were adulterers then all Jews are bastards since they descended from the patriarchs and specifically, Israel, the polygamist Patriarch of the Israelites. By such a lose rendering of scripture, Jesus Himself, would be a bastard through the lineage of Mary his mother who was also descended from the Israelite patriarchs. The fact, however, is that David was accused of committing one case of adultery, the case with Bathsheba, another man's wife. The Bible is so astoundingly clear on this that I'm shocked that there are those who don't want to accept the Bible's definition of adultery but I'll continue to try to help with my answer here.

In 1 Corinthians 7:2 "let each man have his own (heautou) wife, and let each woman have her own (idios) husband." lol, now remember, it doesn't say "idiot husband" it says "her own" for which the Greek word is "idios."
The Greek reflexive pronouns are different for "his own" and "her own". The reflexive pronoun for "her own" is "idios" which means unique to her, just o­ne, as in the following sentence when referring to the city of your birth of which there is just o­ne.

Luke 2:3 And all went to be taxed, every o­ne into his own city. (his own - idios = o­ne to each person)

The Greek reflexive pronoun that is used in "his own" as in "his own" wife is "heautou" which means his own in the sense that you can have more than o­ne as in the following sentence.

Philippians 2:4 Look not every man o­n his own (heautou) things, but every man also o­n the things of others.

You can clearly have many things. This is why heautou was used here instead of idios.

The Greek words in 1 Corinthians 7:2 give a clear example of where a man can have more than o­ne wife but a woman cannot have more than o­ne husband. Keep in mind that what needs to be understood here is that it is of no consequence whether the Greek word for wife is singular or plural, the Greek word "heautou" leaves open the door for more wives. It would be illogical to use plural here. If it were plural then it would be saying that every man should have more than o­ne wife which is clearly an impossibility unless God starts making tons of women. Ephesians 5:28 - 31 falls under the same logic. If the word wife were plural then it would be inferring that every man had more than o­ne wife. Of course it would not say that because even in highly polygamous societies, less than 15 percent of men have more than o­ne wife.

1 Corinthians 7:12-16 has no bearing o­n the number of wives that a man may have but if it did it cannot be taken as scripture for it is prefaced with:1Corinthians 7:12 "But to the rest speak I, not the Lord:"

It is true that marriage is between o­ne man and o­ne woman. David was not married to a bunch of women as if they were a corporation where it was between David and the corporation. David was married to each of his wives, individually.

No man is required to have more than o­ne wife. However, we are all required to understand our Bibles and not to change the meanings of words. The Bible, not you nor I, define the words adultery and fornication. They each have their own specific meaning. If our fellow Christians don't know these definitions then we must show them the verses. Please read the following article for a more comprehensive discussion of fornication.

What Is Fornication?