Paul believes that joining to a harlot is wrong.
"Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make
them the members of an harlot? God forbid. What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh. But he that is joined unto the Lord is one spirit."
(1 Corinthians 6:15-17).
Jesus view of marriage is about one man and one woman joing together (Where two fleshes become one). In other words, the intimate act between a man and a woman is the joining and coming together as one flesh. If Hosea joined with an harlot, he would be joined to all the other men that she slept with. For sex is only to be within the bounds of marriage and it symbolizes the joining of two into one.
Also, God's law on sexual purity in marriage is also equally important to consider, as well. If a man found a woman to have lied about her virginity, he was not bound to marry her. See, the whole point of marriage was for a woman to be faithful before that marriage so as to dedicated to him. Are their exceptions to the rule. Yes, I believe Rahab is a special case because she repented of her sin. She was not still a practicing prostitute when she married. For explaining that there is an important spiritual message so as to make a point to side step the issue of morality in this situation does not change the morality. It is still wrong if believe God told Hosea to marry a practicing prostitute. For what fellowship does light have with darkness?
1 Corinthians 6:13-15 is talking about fornicating with a harlot.
v. 13 said:
Meats for the belly, and the belly for meats: but God shall destroy both it and them. Now the body is not for fornication, but for the Lord; and the Lord for the body.
God also ordered a brother to marry the deceased brother's wife if the deceased brother's wife had no children by the deceased brother. And God also permitted priests to marry the widows of other priests. In my opinion there is no bond in marriage except to your spouse. You're not being joined to the woman's past husbands and lovers. However, outside of marriage that may be another story.
The fact that Gomer had to be purchased back from another man by Hosea even after she was his wife shows that she was committing prostitution and adultery after they were married. Hosea would not have to buy her back if she were worshiping idols.
Hosea 1:3 So Hosea married Gomer daughter of Diblaim. She became pregnant and gave birth to a son for Hosea.
Hosea 3:2-3 So I bought Gomer back for 6 ounces of silver and 9 bushels of barley. Then I told her, "You must stay at home with me for many days.
You will not be like a prostitute.
You will not have sexual relations with another man. I will be your husband.
It's clear that her prostitution was not idolatry but sexual relations with other men. But to be honest, I think a woman becomes something other than a prostitute when after she is married she fornicates again with another man who is not her husband. She becomes an adulteress. In the presence of sufficient witnesses she would have been executed. Whether or not the prostitute continues to be a practicing prostitute is up to her. Otherwise she's a married woman who belongs to one man. The Bible is clear that once married the only way to stop being married is for one partner to die. Execution of the adulterer/adulteress accomplished that in Old Testament times. But in the absence of such measures you were still effectively married in my opinion.
Whatever Paul says regarding this seems to be about fornicating with prostitutes (i.e. using the members of our bodies to join with the members of prostitutes' bodies). I think sex definitely is a big part of how we define marriage, because those who had sex with others in the Old Testament were supposed to marry them afterward. But sex itself is not marriage. We also have to keep in mind that Paul is speaking to Christians - not to unbelieving Jews like John the Baptist used to be. John the Baptist was a great Jew, and he was used by God. But he was not initially a child of God. And there's no guarantee Hosea was either.
In summary I think it's clear that Gomer was a prostitute, that the Torah did not explicitly teach against us marrying women who were prostitutes in their past (whether or not they repent is up to them, but presumably they're not prostituting themselves at the same time they're getting married to us), and that God commanded Hosea to marry a woman who had prostituted herself in the past and committed adultery later in their marriage.
What's more, Paul doesn't just only teach about the commandments of God. He goes a step further and teaches about the spirit behind those commandments. For example, one of the responsibilities he placed on Gentile believers was to not eat food sacrificed to idols. However, that is not a command explicitly found in the Torah. But it is often condemned in the Old Testament. And if Paul is teaching others to not even marry women who were prostitutes in their past, then that teaching of his is probably wise advice for every child of God. And a person would probably be an idiot not to follow that advice. But it is not an explicit rule of God's, and if God orders one man (i.e. Hosea) to do otherwise to be as an example to a whole nation, then I think God knows what he's doing.