What you are talking is social. Try to ask this question to different people of different countries. You will get different answers.
If you talk biology then Sex is only for reproduction and nothing else.
In short whatever you are referring in the question is according to the society around you. Same social situations may not be there somewhere else.
For example... Japan, China, India, Middle East, Brazil, US, France and UK have different social situations.
About relationship. It is purely immolation. What you call emotions is nothing but a hormonal change. You must have never felt a need for relationship when u were 4 year of age neither when u were 8 years of age and nor at 10. So why do you feel it now.
for further information you can go to ishafoundation.org and in videos section watch the video "is marriage the right path..?"
You will find your answers....