This has been brought up numerous times before but nonetheless it's still good to refresh a debate/ argument.
We live in a society where heterosexuality is the norm and where maleness is better than femaleness. There's no point attempting to argue one's way around this. Put simply: if a girl acts masculine or is a tomboy, it is viewed as cool and is generally not frowned upon, if a boy is feminine it is. This says everything you need to know. Thus a girl "acting like a boy" is a good thing and a boy "acting like a girl" is a negative thing. It is strange considering how quite a lot of LGBT culture concerns itself with cultural Marxist constructs (i.e. feminism) and attempting to bend and twist traditional notions of gender still pertains to the ideological view that masculinity = good, femininity = bad.
However, it is true that I must provide a counter-argument to this. This isn't always the case. Quite a lot of people couldn't give a shit.
It's also worth noting that LGBT culture is a culture defined by sexuality. In simple terms, some people don't find femininity attractive - thus it isn't necessarily always about hatred to feminine people but simply: I'd rather put my cock inside someone who was beefy, muscular and played football.