Laertes and Ophelia have this first discussion in Act I, scene iii. Laertes is getting ready to leave and gives Ophelia some brotherly advice in these words:
For Hamlet, and the trifling of his favours,
Hold it a fashion, and a toy in blood;
A violet in the youth of primy nature,
Forward, not permanent, sweet, not lasting;
The perfume and suppliance of a minute; (10)
Laertes is saying that Hamlet views Ophelia as just one of his many women. He is also showing that this is natural for a man of Hamlet's age and status. To him, Ophelia is just a toy.
Polonius tells Ophelia that the advances and offers that Hamlet has made for her love and affection are false:
Think yourself a baby,
That you have ta'en these tenders for true pay,
Which are not sterling.
He notes how immature and ignorant Ophelia is. Furthermore, by the end of their discussion, he commands her not to accept any of Hamlet's advances from this point forward.
These man go to the trouble and effort to warn Ophelia because they know Hamlet and are worried that the difference in their social classes will make a relationship difficult. They are also concerned that his drastic emotions would affect her. These are two protective men.