Boys still have the relentless pressure. There's just less of a social trend to try to change it, so people don't even see it. Act a little weak? Get ready to be tossed in the trash can. Interested in books instead of football? LOL!. God forbid your favorite color be pink. And it also starts very, very early.
It comes from everywhere. Advertisement, TV shows, friends, school... you can't avoid it. And then there's probably SOME biological factor...at the end of the day we're just very complex chemical reactions-based machines, but machines just the same. No sane woman would agree to go through giving birth without some pretty strong biological programming/instinct.
I think the "trick" isn't to fight it, but to embrace it with some steering. There's a lot of unisex toys and crafts that will fall into the silly gender bucket, but steer them toward more diverse options. Think Lego Mindstorm for example. And if they like dolls, get them both the ones advertised to them and action figures (if you try and only buy "boy toys", it backfires because they want what they cant have...but if you get everything, they'll stop caring about the difference and make their own choices...). If they like cooking, you can encourage them to experiment, which ends up being chemistry.