"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them."
He's speaking about Jewish Law, not Roman Law, and he's speaking about Jewish prophets. Unless you completely ignore what he's saying, he's explicitly stating that Jewish Law is still in effect, and that his teaching are *in addition* to what his Father has already laid out. Note that The Father/Son thing isn't really accurate, either, because the Trinity is One, so it's really his own law via his other manifestation.
The *only* piece of Jewish law that he specifically overrides is performing sacrifices, as he states that he is the final ultimate sacrifice from now until Judgement Day.
So if you're a fundamentalist Christian (for example, a Calvinist) you adhere to the Ten Commandments because that's just as important as the direct word of Jesus, because it's the same god commanding them. This extends to the other lessons of the Bible. Being gay will get your whole town obliterated, so don't do it.
You cannot claim Christianity and then only use select passages to back your specific interpretation. Either you believe the *whole* book is the inspired message of your deity, or you're just engaging in a self-affirming tautology.
So you are 100% incorrect, unless you wish to parse words like the pharisees that Jesus so notably denounces.