The actual Biblical usage of "Legalism" is relying upon self to either attain or remain 'righteous'.  It has nothing to do with a church (or person) holding someone responsible for their 'unbiblical' behavior.    Crying "legalism" doesn't let a person off the hook to obey clear biblical teaching of expected behavior. 

It is the 'false righteousness', or 'self-righteous' behavior of the Pharisees that Jesus was pointing out in His encounters with them. The Pharisees thought that by their strict adherence to "the traditions of the fathers" that made them attain 'righteousness'.  In fact, the believer's righteousness is given to us "by Grace, through Faith", not by our good deeds or the "do's and don'ts".  That gift is also how the Apostle Paul contrasted 'Grace' & 'Legalism' to the Galatian & Ephesian Christians - righteousness comes by Grace.  And then after Grace, and by Grace, we turn our lives over to God to live according to His Word.   But we cannot confuse obedience with legalism.  Jesus didn’t and Paul didn’t. 

Obedience to God's Word because we've received God's Grace, in contrast to 'legalism' to attain righteousness, is commanded and even expected by Jesus ("if you love me you will keep my commandments"; Please read all of John 14:21-24).   Likewise, we are commanded to obey throughout both the Old and New Testaments. It is not ‘the Law’, but rather ‘obedience in our love of the Lord’ which Jesus teaches in that passage. The whole world, saved and unsaved, is accountable to God for their obedience or disobedience.  Even the unsaved have His law ‘written upon their hearts’, and are accountable for it.  How much more a Christian who has the Holy Spirit dwelling within? 

While ‘Legalism’ is applying human works to ‘attain righteousness’ in order to be pleasing to God, 
‘Obedience’ is in the power of the Holy Spirit, and is the proper and pure behavior described in the Bible.  And it is in loving response to God’s goodness.  

 

... you can count on them to offhandedly refer to the 'Red Herring' of Legalism to distract you

When talking to someone who is getting ready to do what they want even though they know that the scriptures teach the opposite, you can count on them to offhandedly refer to "Legalism".  But of course, they are missing the real meaning of 'legalism', but in terms of 'logical fallacies' they are creating a "Straw Man" or a "Red Herring" to distract you.  They equate the requirement for obedience in a matter addressed in Scripture with the common (popular) definition of "Legalism".  Their assumption is that "we all know that Jesus condemned legalism".  Hence, they can do what they want.  

Grace and Forgiveness
In the Old Testament and New Testament God's grace, mercy, and forgiveness is offered to those who have sinned.   Probably the most sensitive issue today is Divorce & Remarriage.  Those who have divorced illegitimately cannot claim "Legalism" to negate Biblical principles, nor the clear, straightforward (and unqualified) statements of Jesus. Jesus ‘ministering grace’ and his forgiveness of the woman taken in adultery ("neither do I condemn thee") cannot be taken as a contradiction to what he said Himself about marriage, divorce, and adultery.  Jesus was not being ‘legalistic’ in Matthew but then gracious in John - He does not contradict Himself.  Neither does forgiveness contradict Jesus’ own statements on marriage and divorce. God’s grace, mercy, and forgiveness are reflected throughout scripture, both Old and New Testament, and there is ‘no condemnation’ to those who are in Christ Jesus. We see multitudes of sinners and righteous alike flock to Jesus, and he taught them. Some of that teaching was difficult, even for the disciples, but he declared all that the Father had given him to say. Does that mean that those who have divorced and remarried illegitimately are without forgiveness. In no way.  But there are consequences. Compare David’s sin and forgiveness. But the forgiveness does not imply that further willful transgression is acceptable, and is to be overlooked. "Shall we sin that grace may abound?"

Grace, Truth, and Love
Jesus is full of Grace ‘and’ Truth, and they do not contradict. Likewise, the ‘full counsel of God’ does not make us choose to obey one and minimize or explain away the other. Jesus loved the rich young ruler, but he gave him a straightforward statement that couldn’t be misunderstood. "Go sell all that you own and come follow me." That young man fully understood, and it was too hard for him. He didn't accuse Jesus of being "Legalistic" in what He expected of that rich young man.   

Ministry
I believe that Jesus demonstrated how to deal with real people in his ministry of grace, and with his teaching of truth. He never contradicted either. Likewise, Jesus used the Word as ‘truth texts’ to ‘minister grace to the hearers’. Sometimes that grace was convicting, encouraging, correcting, or teaching (how to live). But, sometimes, for some it was too hard to accept. 1 Timothy 3:16 gives the whole range of applications of inspired scripture -  some is for 'reproof', and some is for 'instruction in righteousness'.  All is binding on God's children.

Leadership and Ministry is difficult, and finding the truth to obey sometimes seems to be difficult, but I truly believe that God hasn’t made important principles of life hard to understand. They are merely difficult and painful to apply. Loving someone is wanting the best for them, and the best for them can never be contradictory to God’s revealed will. God has declared His love for us, and has called us to obedience of his Word. Even Jesus ‘learned obedience by the things which he suffered’ (Heb. 5:8,9). This is a painful issue, but we are called to discern "Grace, Obedience, Legalism, Phariseeism, Love, Ministering, Forgiveness, and Truth". These must be defined by scripture and not common Christian usage, and we understand that "He has given us all things that pertain to life and godliness" (2 Peter 1:3). Relying upon the sufficiency of scripture for faith and practice is not legalism, but orthodoxy.

May God give us wisdom in understanding His Word, and also give us His grace to obey exactly what he reveals to us through His word.  "Thy Word is Truth"