There are two separate things going on here: sexual orientation and sexual preferences.
Sexual preferences are about your 'type' - dark or fair, tall or short, masculine or feminine, build, eye colour, dress sense, tastes, interests, politics, personality, religion or lack of, attitude to family etc. etc.
We all discriminate all the time based on our preferences, not only in sexual relationships but in our friendships too. This is completely OK. If you don't think this is OK then please describe the alternative in a way that doesn't sound like a Black Mirror episode.
Sexual orientation is about which sex you are attracted to - male, female or both. For some people this is fluid but mostly not. It's a protected characteristic because throughout history both men and women have been beaten, raped and killed for being attracted to the wrong sex. This is still going on today throughout the world, including western countries.
No man has been pushed off a building because he preferred an athletic build.
No woman has been correctively raped because she preferred smaller breasts.