Why don't many Christians follow this: " For all the law is fulfilled in one word... Love thy neighbour as thyself." - Galatians 5:14

I see that this question has already accepted an answer, but the comments suggest that some concerns still remain, so I'll attempt to provide an answer that fills in these gaps.

I would think that most Christians agree that we should love our neighbor as ourselves. The disagreement concerns what is considered to be loving. This would to come down to a language issue. The English word "love" is often used in a variety of ways. However, the New Testament was written in Ancient Greek, which happens to have four words for "love", each concerning a different aspect. I'll briefly describe them here:

• Eros is used to refer to romantic or sexual love. This word is never used in the New Testament.
• Storge is used to refer to family love, such as the love between a parent and a child. It is used only twice in the New Testament.
• Phileo is used to refer to brotherly of friendship love; it is the kind of affection demonstrated between close friends. It does not carry any sexual connotations. This is word is used throughout the New Testament, such as as when Jesus wept over the death of Lazarus:

Jesus wept. So the Jews said, “See how he loved [phileo] him!” (John 11:35,36)

• Agape is used to refer to the kind of love that God has shown people. This includes kindness, goodwill, benevolence, and self-sacrifice, meaning putting others before yourself. It is not just a feeling. It is demonstrated by actions, such as when Jesus died for our sins. It also does not have any sexual connotations. This word is widely used in the New Testament, such as in the Greatest Commandment:

You shall love [agape] the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love [agape] your neighbor as yourself. On these two commandments depend all the Law and the Prophets. (Matthew 22:37-40)

For the whole law is fulfilled in one word: “You shall love [agape] your neighbor as yourself.” (Galatians 5:14)

Beloved [agape], let us love [agape] one another, for love [agape] is from God, and whoever loves [agape] has been born of God and knows God. Anyone who does not love [agape] does not know God, because God is love [agape]. (1 John 4:7-8)

From this, we can see that when God asks us to love each other, He is referring to kind acts; nothing sexual or romantic is implied. But this still leaves open the question of why h*m*sexuality goes against this and is considered a sin. This can be answered using a couple of passages:

But from the beginning of creation, ‘God made them male and female.’ ‘Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.’ So they are no longer two but one flesh. What therefore God has joined together, let not man separate. (Mark 10:6-9)

“Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.” This mystery is profound, and I am saying that it refers to Christ and the church. (Ephesians 5:31-32)

When God created marriage, He made it as a union between a man and a woman. This relationship is designed to be a reflection of Jesus' relationship the church (the church being a term the Bible uses to refer collectively to all believers). We must remember that it was God who created marriage and not us. Any deviation from this form of marriage would go against God's design and would be an act of unkindness towards God, and therefore sinful.

It should also be noted how this relates to judging others. Let's consider what Jesus said regarding this:

“Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. Why do you see the speck that is in your brother’s eye, but do not notice the log that is in your own eye? Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother’s eye. Do not give dogs what is holy, and do not throw your pearls before pigs, lest they trample them underfoot and turn to attack you. (Matthew 7:1-6)

God has asked us not to judge, but this does not mean that there is no difference between what's right and what's wrong. A speck is still a speck, and a log is still a log, and Jesus says both ought to be removed. There is still a difference between pearls and swine, and Jesus says that they don't mix.

To further clarify that h*m*sexuality is considered a sin, the Bible make an unambiguous statement:

Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice h*m*sexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God. (1 Corinthian 6:9-11)

I'd like to conclude by bringing your attention to the last part of this passage: we all were born sinners. I will not deny the h*m*sexual feelings that people have as being something that they have control over. All of us, every single person, is born with a sinfully nature that we cannot necessarily control and is opposing to God. But if we desire to follow Jesus, we must deny ourselves, take up our own crosses, and follow Him. God has promised that anyone who truly believes in Jesus will be set free from their sins. We may not become sinless all at once; it usually takes a lifetime. Ultimately, this is what the the church is: a group of sinners who have trusted in Jesus for their salvation. All of us are sinners, and we should not judge or considered others to be more sinful than we are, whether they are greedy or drunkards or h*m*sexuals or anything else. We should be accepting of everyone and help each other the best we can to grow closer and closer to God.