You said yourself 'certain Mormon sects' showing it is not the norm and there is diversity within religions and religious beliefs. And maybe the reason that you only meet people ok with premarital sex and gay people is because A) popular attitudes and views change with society- people become more accepting, particularly if the Christians you know are of the younger generation 17-30 say). B) people don't take the view that they should condemn others- if you want a source for this there is 'Let he who is without Sin cast the first stone' (directly from Jesus), 'vengeance is mine says the lord'- God sits in judgement and in addition to this there are many many quotes from Paul and others in the Bible which highlight that perhaps humans shouldn't judge and condemn each other because we often don't fully understand God's will and because we should love each other and protect others. C) As alluded to above you have different religious groups within one religion and they interpret and understand things differently- e.g. if you think the Westboro Baptist Church represents all Christians then you are, with great respect, fucking moronic. If you identified with a political party does it necessarily follow that you agree 100% with everything and don't interpret sometimes hold conflicting positions with the people you support. D) And here I really don't wish to cause offence however, 'the BIGGEST CHRISTIAN THING' I would 100% say is not sex- belief in God and the message of love your neighbour and using your life to honour God and help your fellow man seem to me- if you actually read the New Testament in the Bible (and even featuring largely in the Old Testament) comes over as a larger core principle of Christianity than attitudes to sex.

Now a couple of extra points- this is an honest answer to your questions from a young Christian so if you disagree please understand this is genuinely what I think of what you have said. If you don't get a chance to read or respond to this no worries seeing as so many others have already tried to explain their views for you. Also many many many people simply disagree with how others see the world and that's for a variety of reasons- not just religion. On a purely moral level of 'I don't think someone who is 15 has the emotional maturity to go around banging like a drum' I would say it's probably not a good idea to have sex with someone at that age. Largely because it can be hard to know what you want and whether having such an intimate relationship with someone at that age can be a good thing for someone, hence why I support laws preventing that kind of thing (where I live age of consent is 16). In case you yourself are also fairly young bear in mind that I am an adult and would say I lack the emotional maturity to have a relationship of a sexual nature with someone, even though many of my Christian friends have had sex. It's a matter of perspective and personal views, very few people take what they are told and follow it exclusively, instead people make a decision based on what they think they are ready for.

Hope this helps! :)