Well I do agree that there are different degrees of sin - stealing is not considered as serious as say murder - yet all are the same when it comes to falling short of God.
I do agree with drawing clear standards of what is right and wrong, and the purpose of the law was to show that, to show everyone that this is what God accepts and pleases him and that is what God detests and hates.
I don't agree with the scenario described in regards with not associating with them. This is not a personal comment, but the description admittedly sounds much like the teachers of the law who chose not to associate with 'sinners' - compare this with how Jesus regularly hung around with tax collectors and 'sinners' and spoke with love to them. He befriended them and because of his love for them they were won over.
I don't believe christians are guaranteed to be influenced by 'sinners' - they will only be so influenced if the Spirit is not full in a christian. A person who is full of the Spirit and power will actually influence those around them as salt and light. If a person does not spend time being replenished in spirit then they begin to operate on empty and they will be more susceptible to influence by the devil.
Christians cannot remain separate to the world and somehow love those people hoping they will be won over by just hearing a bunch of words. Jesus engaged with the lost and was not afraid of being around them, while the pharisees closed themselves off and only condemned them by holding the law against them as a barrier to salvation. Paul too became a gentile to the gentiles that he might win some over. He engaged not only with the local synagogue but also the marketplace. They even had to fight people making them into some of their gods at one stage! If Paul had not enagaged with people and been willing to be amongst them to share the message, then he would've been as inaccesible as the pharisees.
Yes, we guard our hearts and our minds. But we have to engage in the mess that is the battle. Our battle is not against flesh and blood [e.g. homosexuals, thieves, murderers, etc] but against the principalities and powers of this world. Therefore we are not to fight people but be prepared for the spiritual battle that goes on, the spirit that is behind a person. And we cannot engage in battle unless we go to where the 'sinners' are.