Very simply, queer theory is based on observing that, in the West through most of history, people have expected to grow up, get married, and have children. And they expect their children to do the same, and so on through the generations. That pattern is very familiar to most of us, and we (consciously and unconsciously) use the idea of the heterosexual family that reproduces itself as a model for lots of other things.
When queer theory began to come into being, homosexuality was something that was held to disrupt that pattern. Obviously it's less so now, as there's more legal and social acceptance of gay parents. But, even very recently, a common response to a person coming out to his or her parents was 'but I wanted grandchildren!' or 'but you'll never be able to get married!'
So, homosexuality disrupts that familiar expectation of growing up, marrying, having children, and seeing them repeat the same pattern. And in turn, it lets us think about other kinds of disruptions to familiar patterns. Queer theorists talk about 'queering' expectations, meaning disrupting them, in similar ways to the way homosexuality disrupts the pattern of marriage and children.
There is more to queer theory than this, and sure, it does relate to gender, but at the very basic level, this is it. Just a way to think about disruption and social expectations. Not scary.