Jesus dinned with sinners. Correct. I'm glad He did or I'd have lonely meals.
He still said Sin no more. He never condoned our sins.

Marriage is a Sacrament, a covenant, that cannot be broken as you said. But Jesus also very clearly articulated marriage between a man an woman in Sacred Scripture and Sacred Tradition. "He didn't make a treatise on the definition of marriage" He did, here is part of it.. you can read more in the Catechism:

THE SACRAMENT OF MATRIMONY
1601 "The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament."84
I. MARRIAGE IN GOD'S PLAN
1602 Sacred Scripture begins with the creation of man and woman in the image and likeness of God and concludes with a vision of "the wedding-feast of the Lamb."85 Scripture speaks throughout of marriage and its "mystery," its institution and the meaning God has given it, its origin and its end, its various realizations throughout the history of salvation, the difficulties arising from sin and its renewal "in the Lord" in the New Covenant of Christ and the Church.86

So with the authority Jesus bestowed to compose infallible doctrine for us, He did in fact write it into law defining marriage for us. "a text above the Master who interprets it for all human beings, then and now" God is the Word, the word became flesh via Jesus, and then the Word became written via apostolic succession. The Master appointed and chooses the human beings who interpret it for us. His kingdom is built on a hierarchy both here and and in heaven. It's the only way to have unity and Full Communion with Him.

In terms of being welcoming and being generally kind, compassionate, and not judgmental, I have no quams with how you understand Jesus. But don't think God is a father without rules. SJW seeks to violate God's laws and instructions on how to live a moral life and get your soul into heaven. Condoning sin means you don't care about your neighbor's soul as long as he/she is happy and comfortable in this life. You were made for greatness, not comfort. We are all called to carry a cross.