I think it’s a valid question, and I don’t think you should be downvoted actually. Now, I hope you do read my answer.
The answer is that some components of gender are socially constructed — like what different genders wear, what roles they fill, and certain behaviors. However, there is also very likely a component of gender that is biological and doesn’t always manifest itself in your bodily form or sex organs, but in the brain.
We know from genetics that it’s common for mutations to occur. We also know that certain chemicals and hormones impact brain chemistry, especially during gestation. So, why is it so far fetched that how one thinks of one’s self in their mind might not align with their bodily characteristics?
Sure, on the whole, people who identify as masculine in their brains tend to have a penis and tend to be physically attracted to those who present outwardly as feminine and have vaginas. And those who identify as feminine tend have vaginas and tend to be physically attracted to those who present outwardly as masculine.
But, nature, genetics, and hormones can result in those three components - gender, sex, and sexual preference - having different combinations. Trans folks might have a physical body that is different from the gender they feel they are in their mind. They might feel relieved to bring these two things into alignment, and even to adopt some of the socially constructed components of a gender that they identify as in their mind.
We also know from the existence of gay and lesbian people that sexual preference does not always align with one’s sex or gender. It is possible to identify as a man, and to be attracted to men. It is possible to identify as a woman and be attracted to women. Why? Well, nature, genetics, and hormones can some times create people who have small differences from the average population.
That’s all it is and the existence of these types of people poses no real threat to you. Nor are their differences from your own a valid reason to deny them equal treatment under the law, nor privacy in the medical decisions they make to feel more like themselves.