Heterosexuality is just so common. Homosexuality occurs in 5% of the population, which admittedly is a big number, but it is still out of the ordinary. So a lot of people don't understand it. If an adult doesn't understand it, he cannot properly explain it to a child. Additionally, when a child sees that two men own a house together and they have the same bedroom, he might wonder why that is the case when it's normal to this child that men marry women.

Sexual orientation is who you are sexually attracted to. Yes, people form relationships for many different reasons, be they financial, a desire to have children, interdependency or just doing what other people are doing. Romance wasn't always a factor in who married whom. But you wouldn't necessarily make a life long commitment to someone if you didn't want to have sex with that person. So yes, heterosexuality, homosexuality, and bisexuality all come down to sex. You cannot talk about sex to a child. You can tell the child that his two uncles live together because they love each other the same way his dad and mam love each other. But it is very complex for a lot of people and this requires a lot of thought and patience.