Simon17,
When I say that God is the source of objective moral values, I don't mean to say that every moral issue has a clear-cut answer that we can apply in any given situation as if God had given us a rule book that covers every conceivable situation. I would also acknowledge that God is the final judge, not me or my opinions.
That said, here's how I view what the scriptures say regarding what you asked me about.
1- Homosexuality
The scriptures say that any sex outside of marriage (marriage between one man and one woman) is wrong, whether it is heterosexual or homosexual.
2- Abortion
The scriptures don't specifically discuss abortion, but given the basic reverence for life and references to life in the womb, I believe that abortion is not a biblically acceptable method of birth control, although I think it would be allowed if needed to protect the mother from death or great bodily harm just as self-defense is.
3- Slavery
Paul's letter to Philemon doesn't directly condemn slavery, but I think it would be impossible for Philemon to follow Paul's counsel and continue to be a slaveholder.
That said, I am aware that God allowed for slavery in the OT, especially in cases where Israel conquered other nations in war. Since they didn't have the capacity for prison camps, it would seem that slavery was an alternative to utterly destroying the conquered enemy.
4- Genocide
Obviously, the Bible condemns murder. God, of course, can take life when He chooses since He is God (He does this every day). I am aware that in the OT God commanded Israel to utterly destroy certain groups of people (the Amalekites, for example). Only a clear and direct command from God would make such a thing morally acceptable, and God would only command such a thing if it fact it was morally acceptable (God being the ultimate judge). I am also aware of the danger that various tyrants may claim or even believe God has given them such authority (radical Islam, for example).