How is Romeo presented as a insincere lover in Act 1 Scene 5

Prior to this scene, young Romeo has been pining away for Rosaline, a woman he loves but who, apparently, does not return his feelings. He's been mopey and reserved, keeping to himself and worrying his parents with his extreme melancholy. He didn't even want to go to the party at the Capulets' house, but a mixture of his friends pestering him and fate leading him compelled him to go.
Once at the party, he sees Juliet. He is immediately enamored of her, asking,

Did my heart love till now? Forswear it, sight,For I ne'er saw true beauty till this night. (1.5.59-60)

Up until just a few moments ago, Romeo had sworn that he was hopelessly in love with Rosaline. However, it seems that one glance at Juliet has completely erased that "love" he felt, and he claims to feel true love now. This certainly makes him seem like an insincere lover. He doesn't know Juliet—he's never spoken one single word to her—but her appearance has, in his mind, caused him to utterly forget the one he so "loved" before. This makes it seem as though it wasn't really love that he felt before, and if it can be thrown over so easily now, it seems as though this new love could come and go just as quickly.