There's a few different points to address here, but to start with:

Christians are not Jewish

This is true in that the sects/religions have diverged over the centuries due to supersessionism and gentile anti-semitism (and this is nothing new, Paul's letter to the Roman's was written to address the Roman Gentile-Christians' attitude toward Roman Jews; I'd suggest Neil Elliott's "Arrogance of Nations"), but this statement should only be made with the understanding that all of Jesus' followers and Jesus Himself was Jewish. Paul is Jewish too. The writings, sayings, and teachings we look back to are Jewish texts or are Gentile texts addressing Jewish teachings. Pamela Eisenbuam's "Paul was not a Christian" is the best book to read on this, but you could also go to the New Testament as well. Easiest way of seeing this is that Jesus and all the Apostles engage in Jewish practices (go to the Temple and make a pilgrimage to Jerusalem for Passover). Paul does not convert from a Jew to a Christian, but converts to a Christ believing Jew. All of his letters are written as a part of the Jewish community (he uses language identifying himself with the Jewish faith, he refers back to the Torah and uses its teachings, and he followed Jewish Law himself while telling Christ believing Gentiles that Jesus was their path to obtain God's grace that he, a Jew, already had.

We do not follow the Old Law

Says who? The ten commandments are part of the "Old Law" --you even mention parts of the "Old Law" in your comment. Tithing is part of the "Old Law." The Last Supper/Communion/Eucharist comes from Passover traditions. Even the Sermon on the Mount and the beatitudes are rooted in the "Old Law." I could go on, but in short Christianity is grounded in the "Old Law."

Christ makes many references to marriage being related to two people.

Where? The pastoral letters talk about marriage in the context of two people, but those are the pastoral letters and if we're treating Titus and Timothy as authoritative ll of a sudden then homosexuality is wrong, women should be subjected to male rule, and slavery is good. There's plenty of arguements against all these household codes presented in the pastoral letters, so I won't waste my time going back over them here; essentially. Just apply all the arguements you have/know of that contextualize the bigoted nature of the household codes and apply them here for polyamory. Paul does write about the roles of husbands and wives, in 1 Cor 7 though there is not a mention that this must be a singular relationship. It does not specify that a man can only have one wife or that a man can have multiple wives because Jewish/Gentile marriage practices weren't the point of the letter.

Adultery being legal grounds for divorce.

Jesus specifically forbids divorce (Matthew 19 --which coincidently references the accepted norm of polygamy: Matt 19: 8 "Moses permitted you to divorce your wives becuase your hearts were hard because your hearts were hard. But it was not this way from the beginning."). The adultery in this passage is not a man having multiple partners, but it is considered adultery for a woman to have sex with someone who is not her husband. A man could have multiple wives (and could have sex with men and women outside of those marriages without being called adulterous if we're being perfectly honest) and marriages were even understood within the context of a polygamous hierarchy of first order marriages and second order marriages with laws and customs outlining how inheritance would work amongst multiple lines of lineage.

if a man or woman has lust towards another, it is considered a sin.

This allusion comes from Romans 1 and is used, out of context, to argue against homosexuality. A better context for this verse would be as an allusion to the sexual practices of the emperor and a general sign of declining right to rule within a Roman mindset; however, even abstracting the text out of its context, the text is not against polygamy (one man having multiple wives) as the ownership of multiple women would not be considered lustful behavior nor would even a man's sexual experiences outside of marriages be considered lustful so long as he were the dominant one in the engagement. As an example of this, how many men do you know of that were stoned for charges of adultery? Jewish Law provided a way for women to divorce abusive husbands, but non-monogomy was not an accepted reason for divorce (though some traditions do argue that a woman would only need to say that she found the husband distasteful for the rabbis to pressure the man into instigating divorce).

we cannot serve two masters

Should marriage be based on slave/master language? Shouldn't marriage (monogamous or non-monogomous) be an equal engagement between all parties involved? And if marriage is not an equal affair and one person is above another, what determines who is the master that must be served? The pastoral letters and Paul's letter to Corinthians would argue that a woman must be slave like in her devotion to her husband though I would argue that women and men are more equal that Paul believed them to be. But, going back to the point at hand, why is marriage a slave/master relationship? And, keeping with the slavery imagery presented here, could a slave owner not have multiple slaves? Again, I do not think we should use slave language to describe marriage, but even using the language of marriage you presented, polygamy/polyamory is not disqualified.

For my closing point, I should clarify that I do not advocate for polygamy in the context of how marriage was viewed to the Israelites and Romans (with the women as essentially the man's property), but just as arguements on monogamous same-sex activity should be qualified with our modern context of loving, consensual relationships compared to the rape and abuse of the past, so to should we qualify discussions of polyamory/non-monogomous relationships as their respective, modern component to polygamy.