As a faithful Christian, I agree with the argument that if God says it is a sin, then it is a sin. In other words, I agree with the argument “because God says so.” Like Paul says in Romans 9:20, “But who are you, oh human, to talk back to God? Shall what is formed say to the one who formed it, Why did you make me like this?” Bottom line, God is God and we are not. Good luck trying to convince God that you are right and He is wrong on moral issues.
But… It is still reasonable to ask the question, why a particular thing is sinful. As a believer, this is not required, but it is helpful, and for a non-believer, it is perhaps necessary. So, we can look at your two examples.
You propose that premarital sex hurts no one, and therefore, it is reasonable to ask how it could possibly be hurtful. I say that premarital sex is hurtful for a wide variety of reasons. First of all, such premarital sex is almost never the only such sex. God created the sexual relationship to be an intimate sharing of body and spirit between just two people. Premarital sex in almost every case is a violation of this plan. I cannot speak for everyone, but nearly all people I have ever met who had premarital sex, whether they thought that he or she was “the one” or not, the result was sex with a variety of people. This always produces comparisons, future jealousies and all kinds of problems. Besides, premarital sex, even if only with the eventual marriage partner, also causes hurt. If I cannot show self-control before marriage, who is to say that I can show self-control after marriage. Faithfulness and chastity before marriage creates trust and security after. Here is a question: Who is hurt by obeying God’s commandment of sex before marriage? Answer: no one. Is anyone hurt by it? Yes, every single participant in premarital sex, whether they see it that way or not.
What about homosexual relationships. Who is hurt by this? The answer, again is all those who participate in it. Just one look at the human anatomy and physiology will tell you what kind of sexual relationship is “natural.” Being attracted to a person of the same sex does not hurt the one we are attracted to, but engaging in this activity does. So, same sex attraction is perhaps not sinful, but engaging in a kind of sex which is not that between a man and a woman who are committed for life is sinful and it is harmful. If people do not agree, then they are wrong.
I agree with the statement that these things are sinful “because God says so.” However, common human experience tells us that there is a reason that these things are sinful. The reason is that it hurts us and it takes us away for the healthy thing that God created for us to do.