I am not Christian, but I grew up Christian. From what I see, scripture can be used to justify just about anything.
As an example, some people will talk about wanting the traditional or Biblical definition of marriage in debates on gay marriage. They may quote portions of scripture that support the version of marriage they would like to exist. However, they ignore all the other scripture that does not support their definition of marriage. For example, an Old Testament view of marriage would have to include raped women marrying their rapist. Women who marry but are not virgins should be stoned to death in front of their parents. Polyandry would be legal but not polygyny.
So people tend to play a game where they quote portions of the Old Testament and portions of the New and mix and match them to fit the views they want to have in the first place. This leads to many, many different possible "Biblical" views as seen by the more than 30,000 denominations of Christianity. (This is probably coming across harsher than intended, but I have seen so many people who hurt themselves trying to follow rules told to them by a preacher or culture.)
So my advise would be to figure out your morality from simpler principles. The Golden Rule (or some equivalent) is a great start. Work from there. Treat others as if you were them.
For this situation, I think you need to settle this before it becomes a big issue. Be honest with Bob and John about what you think and feel. See if you can get Bob to try to understand polyamory. Maybe get a book on it or invite him to talk to people who practice polyamory. A hidden relationship will just poison your marriage.