Is it considered taboo for a girl to touch a guy's ?

i'm just asking this out of curiousity. i just heard one of my girl pals saying this so i wanted to know if this is true or is it just a stupid belief some people have? i mean is it considered to be unsafe or something? or is it linked to some obscenity and all that stuff? and is it ok if a guy touches a girls nipple? or is even that taboo? i am not considering dignity as a reason for not doing that act, is there anything scientific related to it?