In As You Like It, where and how does Jaques shows his conservative idea about marriage in spite of being a cynic by nature?
I think Act III scene 3 is the scene that you should be focusing on. In this scene, Jacques overhears the courting of Audrey by Touchstone, which clearly exposes his own lustful intent and the way he wants to marry Audrey so that he can possess her sexually. Note how the relationship between Audrey and Touchstone makes a mockery of the institution of marriage. Touchstone insults Audrey, calling her a "slut" and predicts she will not remain faithful to him, but yet insists on carrying on with the marriage. Jacques, after making his disdain clear to the audience in a series of asides, emerges from his hiding place and shares his distaste for Touchstone's beliefs about marriage by indicating his own traditional sense of what marriage is. Note what he says:
And will you, being a man of your breeding, be married under a bush like a beggar? Get you to church, and have a good priest that will tell you what marriage is. This fellow will but join you together as they join wainscot; then one of you will prove a shrunk panel and, like green timber, warp, warp.
Jacques therefore clearly shows his belief in the institution of marriage by insulting Touchstone and urging him to go to a "good priest" that can help him understand the true meaning of marriage, instead of going through the sham of the wedding that Touchstone intends to have.