I think it's important to realize, when you really fall in love with someone, you fall in love with a person. You don't fall in love with their gender, race, ethnicity, sexual orientation, physical stature, hair color, etc.
This is important for a lot of reasons.
People change all the time. Knowing that you are loved as a person can give you a great sense of security in a relationship, because you know that the person is committed to you, and not one of the above listed categorical descriptions of you. It can give you great freedom to live life and to explore yourself and your world and not fear that if you change at all, the other person may suddenly fall out of love with you. It gives you a sense of security, because you know that the person loves the ever-changing moving target that is you, not just a static image, the snapshot taken at the moment you met.
It can give a lot of security to a relationship in another sense. It is inevitable that your partner, no matter who they are and who you are, is going to find other people attractive, probably even very attractive in some cases. It's important to realize that finding someone attractive is not the same thing as love. I don't think I really need to describe the difference, and I probably couldn't explain it well anyway, but it's important for both partners in a relationship to remember this, because being aware of this, you won't feel as threatened by outside forces. You will realize that your partner finds other people attractive, and you will realize that this fact has no bearing on your relationship. Similarly, but on a slightly different topic, the fact that your partner is attracted to people who don't look like you does not imply that they don't find you attractive. It's not an either-or situation.
This all opens the world up to you. You can explore, adventure, experiment, you can enjoy yourself, you can learn, grow, mature, change, you can do these things and know that your partner will be there. That doesn't mean it will always be easy - change can be very difficult and can take a lot of work to pull through, but you can do it without the fundamental fear that if you change, you become unlovable.
