Not true, there are very little bilingual households. The reason is that we start learning the other national language in primary school and (in Flanders) we grow up with a lot of English-speaking TV shows that aren’t dubbed, contrary to France, Germany, Spain, Italy, … I myself could speak fluent English without accent at the age of 9, just from watching The Simpsons, Fresh Prince, snooker on BBC, etc. every day at an early age. And since our country is so small, we often travel abroad where we (Flemish) try our best to speak the local lingo, because we know nobody speaks Dutch, again contrary to French and German speakers.
I think it’s typical for young, small nations that are surrounded by larger nations. Apparently Slovenians are also foreign language buffs.