You have to understand that there is a huge difference between God commanding His people to eliminate those who were a threat to them both physically and spiritually and one selfishly taken another person's life for their motives. See, in the Old Testament, God primarily needed to show us that He was God who would reward those who were good and to punish those who were evil. See. Back then God wanted to show that he was a God of justice and righteousness. For can you imagine what kind of evil world we would be living in if God did not stop the evils in this world to spread?
However, marriage has always been a sacred thing to God. It always represented one faithful man and one faithful woman. Jesus even spoke about Biblical marriage was between one man and one woman. If one of those in the relationship was not monogamous, then that would destroy the picture Jesus was trying to make. The KJV says a wife of whoredoms. Plural. The letter "s" at the end lets us know the word is plural. So she is not a wife of prostitutes. She is a wife of a people of a land who had a spirit of prostitution. Meaning, the people of the land depared from the Lord by going after other gods. Whoredoms. Idolatry. This is all in the book of Hosea. Besides, if you believe Hosea was told by God to marry a prostitute, then surely there should be another verse that suggests this. For Scripture always testifies of itself by 2 or 3 witnesses (I.e. cross references).