the simple answer to your question could be that the default or norm is heterosexuality (after all only a small percentage of people are homosexual) and thus this is what we assume of people we meet. We interact with them accordingly, for example by flirting or even going so far as to imagine marriage. It is therefore useful for our surroundings to be able to adjust our expectations. For example (perhaps a poor one), if I am entirely color blind (which very few people are), what would be the use of searching for a beautifully colored present for me? If I assume your hearing is perfect, I would talk to you, perhaps even with my back to you, but what if you weren't? If you know your friend is a devout Muslim, would you invite her over for pork chops? Having different abilities or desires, doesn't change the fact that you are a human and should be treated with the same respect as all other humans, it just means that in some daily interactions we may need to change our behavior towards them.