Okay, so your sexuality is part of your identity. For example, I identify as Pansexual. If you read that last sentence again, its something I IDENTIFY as, which means it relates to my identity. Your identity is in fact an essential dimension of you as a human because if you don't have an identity, then who are you? In addition, your sexuality can also determine your "crowd". Who you hang out with also makes up part of you as a person because it can influence your style, your diction, your opinions and so on.
So, in conclusion, a persons sexuality is an essential dimension to them as a person for a few different reasons, the most important being that it is a part of their identity as a human person.