There is evidence that at just a few weeks, foetuses in utero respond to pain. Therefore, simply from a humane point of view, I disagree with abortion.
While stem cells can be harvested from foetuses, the medical community has also found other ways of growing stem cells which would not take human life.
If you believe that there is no authority higher than us, then really it is up for everyone to do as they see fit. Our society has agreed that we may do that as long as their decisions do not negatively impact other people's rights and freedoms. No one has the authority to call one way better than another. Even our observation of the outcomes could be a completely subjective experience, based on our conditioning, culture, age and sex. However, there isn't really any moral basis or reason why we should care about other people's rights, other than it seeming like a good thing to do. From an evolutionary standpoint, it certainly doesn't seem like a good way to get ahead.
On the other hand, I believe that there is a higher authority. If humans were created by an intelligent being, rather than being a lucky accident of evolution, that has huge implications. It means that our lives (and other people's) are not ours to do with as we please. We are obligated to use them as the one who designed us intended and it is possible that we will be answerable to the one who made us for the choices we make. I believe in Christianity. In my opinion, the Bible is given not to replace people thinking for themselves, but to give some basic guidelines for understanding the world and for structuring society to work effectively. In other words, it gives us the premises to start from and a model for how to work well together but still leaves a lot of scope for new discoveries and ideas. God, being a creative and imaginative person, has created us with endless variety and abilities and functions, yet has made us for a purpose. From this point of view, I think it does matter how we live. It is not that He's just some kind of killjoy that wants to keep us from having a good time. Rather, He has created us to operate best from a certain kind of order and when that order is circumvented, we usually are affected negatively.
I don't think that there is anything remotely wrong with the act of sex itself. From my worldview, if God designed us, it's interesting that sex was designed to be an appealing and enjoyable activity, despite the potential emotional difficulties, responsibilities and pain (eek childbirth!) that it could potentially entail. The problem I see is that sex can't be separated from the way it impacts those who engage in it. (Even if it has no emotional effect, that in turn impacts one or both people!).