Terrorism is the use or threat of violence in an attempt to change political policy. It's not civil disobedience. Sit-ins, protests and the like aren't terrorism. For an act to be terrorism, it must be violent and it must be done in an attempt to change political policy. The acts of the Weather Underground during the seventies were domestic terrorism (although Bill Ayers wasn't a terrorist since his incompetence meant he never managed to bomb anyone except himself). McVeigh was a domestic terrorist.
Most anti-abortion groups confine their activities to civil disobedience; protests, letter writing campaigns, pickets and that sort of thing. That's not terrorism. A few of the anti-abortion groups commit minor acts of property damage as part of their activities, such as using Crazy Glue on the locks of clinics. That's a crime but not terrorism.
There is, however, a faction of the anti-abortion movement which does qualify as terrorists. You've probably heard of them. They go by names like "Army of God" and they kill people to save fetuses. They exist relatively openly, in small but determined numbers and you can find them with a quick Google search. They are exactly the kind of people who were warned about in that report on potential domestic terrorists which the right went nuts over earlier this year (conveniently forgetting that firstly, Bush ordered the report and secondly, there was one on left-wing groups as well). Operation Rescue, while not criminal in itself, frequently acts as an apologist for the terrorist activities of such extremists, in much the same way as Sinn Fein used to do with the IRA. The statement of Randall Terry with regard to the murder of Dr Tiller was largely about how terrible Tiller was. Effectively saying "Yes, it was murder but he needed killing".
Bill O'Lielly on his nightly rant at the world, did several segments on Dr Tiller, publicizing (he claims he didn't invent it) the nickname "Tiller the Baby Killer" and described Tiller's clinic as a "death mill". Other Fox lunatics have said similar things. Their speech is covered under the First Amendment. No-one's disputing that but the First isn't an absolute cover. It doesn't cover you for shouting "FIRE!" in a crowded theater and it's doesn't cover you for encouraging violence. So, how far is too far? Do O'Lielly's comments and those like his cross the line into encouraging violence? I honestly don't know. I couldn't say and the law is unclear.
What I can say is this: O'Lielly and those like him have repeatedly described Tiller and his colleagues as committing murder. They are categorically wrong for several reasons. The definition of murder is "an unlawful killing during time of peace" [my emphasis]. Abortion is legal in the circumstances Dr Tiller acted in, therefore his actions were not unlawful and, by definition, not murder. The Bible might have a different meaning but since the Bible isn't the basis of US law or society (and if you think it is, you are a moron), that's irrelevant. Furthermore, even in those times and places where abortion has been illegal, it has never been classified as murder. A friend of mine (also in the law) and I once deduced that even if abortion was illegal, the only possible legal classification would be "Homicide, sub-class: Illegal abortion".
Personally, I am broadly pro-choice. That's not an absolute position but it is, in general, what I believe. I don't think a fetus qualifies as a person with rights sufficient to overrule those of the woman. As the pregnancy progresses, abortion law becomes more restrictive and rightly so (anyone who describes Roe as "abortion on demand" is either a cretin or a liar). That's my personal view. I have heard all kinds of ridiculous arguments for why I'm wrong, my favorite of which is the gory pictures of late-term abortions (guys, if you can't convince me with a logical argument, you're not going to win with an illogical one) but it's a position that, with only minor modifications, I have held consistently for years now. I know some people who are anti-abortion. A few are the knuckle-dragging misogynists that people imagine but the majority are genuinely well-meaning people who believe abortion is morally wrong. A few are also anti-death penalty and, if nothing else, I have to applaud their moral consistency. I don't agree with them on either point (I am pro-death penalty in the correct circumstances) but their position is clear and morally consistent: They believe that all life is sacred, I respect that.
The difference between pro-choice and pro-life is, for the most part, not a difference of facts but a difference of opinions. Specifically, a difference of opinion about at what point, a fetus becomes a person with the rights thereof. Discounting the lunatic fringe, most people disagreeing over this issue are genuine people with good intentions. Yes, there are knuckle-dragging misogynists in the pro-life camp. I've never met one but I'm sure that there are gleeful baby killers in the pro-choice camp but neither represent the majority. A difference of opinions.
Domestic terrorism isn't a matter of opinion. It's a matter of law and facts. In the US, there is a small but dedicated fringe who are domestic terrorists and they hide within the anti-abortion movement. And they're winning.