Well, morality may vary from culture to culture and it does seem a bit subjective. For quite a while I wondered if anything could really be considered universally right or wrong. As a Christian, I used to beleive that everything good comes from God, that God has the ultimate say in what was good and evil. What I've come to realize lately is that for morality to be universal, there needs to be a common goal, a purpose to it that everyone can agree on. One goal I like is human flourishing. Ethics is based on what will lead to the flourishing of humankind as well as each one of us living a meaningful life. It could even be viewed as a science, not like chemistry or physics, but more like an ecology.
For me, extra-marital sex is wrong because I made a covenant with my wife that we would only have sex with each other. If I were to betray her trust it would destroy her. It would have a terrible effect on my own self-perspective as well. Ultimately, I would sacrifice my family for a few minuets of pleasure. Pre-marital sex has emotional and medical risks, but it is much easier to deal with if the people involved are educated, responsible, and emotionally stable. In fact, pre-marital sex can strengthen the bond of a couple who plan to get married in the future. They will also test their sexual compatibility.
I don't yet have any children with my current wife, but I have a daughter who lives with my ex wife who is turing 8 next month. Like I said earlier, sex requires a person to be educated, responsible, and emotionally stable. I would not allow her to have sex right now, even if she wanted to. Her mother is raising her LDS so I know she'll get plenty of encouragement to be abstinant. I can't guarantee that as a teenager she won't rebel. All I can do is educate her every chance I get and teach her to make wise decisions and to think before she acts. Then, when she's ready, she'll know when the time is right, and if she gets into trouble she'll be able to come to me without fear. As for an exact age where I would absolutely forbid it, I'm not so sure. Sixteen would be the lowest but it could easily go higher depending on her level of maturity and level of responsibility.
As for same sex marriage, I can't think of any reason why they should be excluded. Marriage is for people who love each other and want to be legally committed to each other and be family. People of the same sex are subject to the same rules as those of us who are straight, education, responsibility, and emotional stability. What source of truth? The truths of love, equality, and freedom.
We seem to have taken over Brendan's thread. If you would like to ask me more, feel free to start another one. I enjoy your questions.