Wow that's a lot of questions and will try to answer all of them one by one. :)
Do you believe in love at first sight? Borderline YES! Admit it or not, one's physical appearance is what will attract us first on a random stranger. Someone with an attractive appearance will likely stand out in a crowd making us fall to that person and eventually make us take some action. However over time, what will matter is the compatibility between you two ergo if you can take his/her personality. People who are lucky to have met someone compatible to their personality at first sight usually believes in this kind of love. Love can indeed occur at first sight provided that other person is compatible to your personality.
How do you define TRUE love? I personally think it is considered to be TRUE Love if someone can let go of his/her own selfish desires just to make the other person happy. This usually is what Parents have for their children. They make a lot of sacrifices just to ensure the child's safety and give him/her a good stress-free future. Romantically, I believe if your partner can set aside some of his happiness with regards to yours then it is true love. A good example for this would be one who is "addicted" to smoking/gambling, if one could give up that addiction just for the sake of the other person then no questions ask.
Is it important to have healthy sex relationship in order to develop true love? VERY! Hate it or not, sex is a vital part to ensure a long lasting relationship with someone. Every human has needs that can only be fulfilled by the other person. A "healthy sex relationship" is just the ideal recommended amount for this, not that much and not that lacking just plain healthy.