Well, I would want to start off by first arguing that Leontes does a lot more than merely "question" the fidelity of his wife, and certainly his plans to poison Polixenes for supposedly cuckolding him are incredibly excessive when he has no actual proof at all that his wife and Polixenes have had an adulterous relationship. So, certainly we can argue that the response of Leontes to his suspicions is definitely excessive, especially given the fact that these two kings were supposedly childhood friends. The way in which Leontes can suddenly move from loving Polixenes to wanting to poison him speaks of some defect in his character, as his need to check whether Mamillius is his son certainly highlights. Even if Polixenes had been interested, bearing in mind their long friendship and the fact that he is visiting Leontes, it is doubtful that he would have been so stupid as to try anything at this particular point. On the one hand, therefore, he is wrong to question his wife's fidelity.

However, it is always interesting to consider arguments opposing what you think is true. If we look at this issue from a slightly different angle, we could argue that the way in which Polixenes refuses the offer to stay longer from Leontes, but then so readily gives in to the entreaties of Hermione should actually make Leontes suspect something. Leontes himself mentions that Hermione has only ever spoken more eloquently once, which was when she consented to marry him. Given this, therefore, perhaps Leontes is justified in at least having minor suspicions about his wife and his best friend, even if the extent to which he goes is clearly not justified.