I am not a simple being as the individual that I am. I differ from myself. Why should I presume that changes when I am joined to another person as a new being, enlarged by her presence in me? Why should I assume that being sealed to others and to God will take away whatever it is that makes me the unique person that I am? If I were to lose my identity as an individual, then the web of relationships would itself be impossible. There can be no web if every point in the web is the same. Webs require difference, and sexual difference is perhaps the most important difference in our relationships with each other and with God.
A central facet of my being that sealing recognizes and, indeed, valorizes is my sexuality, because that is what has made the continued enlargement of my being possible through the family. Each of us is a sexual being and our sexuality is essential rather than incidental to who we are eternally. But the female-male difference is too thick to be reduced to a list of properties. I know what it means to be who I am. I know what it means to be related to someone who is unlike me sexually. But I cannot say what it is that Janice's being female brings to our relationship and that my being male brings without risking nonsense.
Too often we fall into nonsense when we try to talk about that. We say things like "Men hold the priesthood and women get to be mothers" as if those were parallel things. So that's another thing I don't know: I can't say what it is that Janice brings to our relationship in virtue of her being a woman and that I bring in virtue of my being a man.
Mormons have a great deal to figure out about sexuality and gender. We have at least as much to figure out culturally and socially as theologically, and there's a great deal that we do not understand theologically. Be that as it may, though, we not only believe but ritualize the importance of sexual difference to Christian life—life before and, with God's grace, with God. Perhaps nothing is more central to Mormonism.