If your primary-school-going son asks you what the meaning on being gay is, how would you explain it to him?

Being completely honest with him. It is perfectly natural to have same sex couples. If you have this discussion with him early he will grow up more comfortable and confident with different types of relationships. If you are worried about wording - explain it to him in the same way that you would with a heterosexual relationship. Being in love should always be celebrated!