There is a difference between assuming an individual is heterosexual if you know nothing about them, and assuming everyone is heterosexual, and hence pretending there are no homosexuals (and others). It is wrong, and damaging to never show or talk about homosexuals, since you're pretending they don't exist.

For example, the majority of the USA is white, and not black. It used to be the case that all TV shows would have fully white casts, and never really have black characters. Sometimes people make this mistake when dealing with homosexuals (etc.)

It's a good assumption to make if someone points out a random person on the street and asks you to guess their sexual orientation.

But in an article about male and female relationships and sexual attraction? It's a pretty bad sign that the author just assumes heterosexuality for everyone, especially when anything else would defeat (or at least greatly complicate) their argument.