“Skip the extraordinary idea, it’s well known to me, as to who in the space of a year, could do more harm with more good luck, Juan Tenorio or Luis Mejia.” 

This quote from Don Juan explains the bet that Don Luis and Don Juan made a year ago. Their intention was to do harm, not just to have fun or seduce women. The bet required that they both seduce women and murder men. This villainous bet is the thing that causes Don Juan's father and future father-in-law to see that he is not a good man.

“I should like to see without them seeing me or being recognized.” 

This quote is from Don Gonzalo. Gonzalo is the father of Don Juan's intended bride. He decides that he wants to know what kind of man Don Juan is and sneaks into the place where the two Dons will discuss their bet after a year. He listens and is horrified to learn what kind of man Don Juan really is.

“Bah. I’ll satisfy you doubly, since I tell you that, just for fun, I’ll add a friend’s lady to the novice nun whom he’s about to marry.” 

Even after he's won the bet, Don Juan decides he doesn't want to have an incomplete list of the types of women he's slept will. Don Luis tells him he needs a novice from a convent. He adds in a friend's lady, which leads to him seducing Don Luis's fiancée.