I agree with your last sentence - that legislation on this issue isn't desirable.
What it comes down to for me is: that adults must be able to (and history has shown, will no matter what the law says) control their ability to reproduce. A woman shouldn't be compelled to have a child because she became pregnant.
What is life?
If you say that a zygote is not a human, then I suppose you must operate on the assumption that at some point in time that this zygote is somehow transformed into human life.
"Life" and "human" are loaded words, as everyone involved with the debate knows. An unfertilized egg is alive - it will die if not cared for by the body. A sperm cell is alive - it will die if not cared for by the body. They, like a fertilized egg, have the potential for being precursors of a child.
You may say that few would argue that an unfertilized egg or a sperm cell is "human life". But, there's religious support for just such an argument in Catholic doctrine. E.g.:
[link|http://www.catholic.com/ANSWERS/tracts/contracp.htm|[link|http://www.catholic.com/ANSWERS/tracts/contracp.htm|http://www.catholic...contracp.htm]]
Contraception
In 1968, Pope Paul VI issued his landmark encyclical letter Humanae Vitae (Latin: On Human Life), which reemphasized the Church's constant teaching that it is always intrinsically wrong to use artificial birth control or contraception for the purpose of preventing new life.
Artificial birth control is "any action which, either in anticipation of the conjugal act [sexual intercourse], or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" (Humanae Vitae 14). This includes sterilization, condoms and other barrier methods, spermicides, coitus interruptus (withdrawal), the Pill, and all other methods of artificial contraception.
The Historic Christian teaching
Most people don't realize that up until 1930, all Protestant denominations agreed with the Catholic Church's teaching and officially condemned contraception as sinful. At its 1930 Lambeth Conference, the Anglican Church capitulated to growing social pressure and announced that henceforth contraception would be allowed in some circumstances. That small crack quickly widened until the Anglican Church completely caved in on this issue, allowing contraception across the board. Since that time, all other Protestant denominations have followed suit, abandoning the historic Christian teaching against contraception and giving in to the permissive mores of secular society.
Today the Catholic Church alone proclaims the historic Christian position on contraception. (Fortunately, though, an increasing number of individual Protestants are realizing that contraception is contrary to the gospel and totally opposed to constant Christian teaching, and they are embracing the Catholic position). Evidence that contraception is in conflict with God's laws comes from a variety of sources:
[...]
If you (the generic you) argue that abortion is wrong, then how can you accept that contraception is OK? Aren't you interfering with the development of a human life? And if you accept the Catholic argument, shouldn't you also agree that every woman should always be pregnant so that the God's commandment to "be fruitful and multiply" be explicitly fulfilled?
I think such a train of argument is sophistry.
At its base, the abortion debate has broken into two sides. One claims to base its arguments on religion, protection of the helpless, etc. The other claims to base its arguments on personal freedom, protection from coercion, etc. Without a common framework there's going to be no consensus and each side will be convinced they're right with little hope of convincing the other.
Cheers,
Scott.