Why does he lie to me?

His probably hiding it to mask his curiousity, and keep you happy.All guys sneak looks at other women this is not un normal , this is normal male behavrior , we cant control them or have them on leashes 24 .7 at least he has the manners to not show or do it around you like my ex.I think unless they are having some physical or secret dates then you really have nothing to worry about its all just harmless talking or looking.