Judicial activism is when the Court issues decisions with no Constitutional justification. Roe was such a decision, therefore overturning it would simply be righting a past wrong of the Court. Overturning Roe only makes law in the sense that it does away with the bad law made by Roe.
My lack of regard for precedent has been formed by such things as I cited before, in how the Court points to it as sacrosanct only when it is convenient. When it gets in the way of some new leftist decision, then to hell with precedent!
As to gay marriage/civil unions; by "or would they just vote not to block them" do you mean to suggest a possible decision whereby the High Court issues a ruling that bans gay marriage? There is absolutely no reason to think that the Sup Court (or any justice on it in even a dissent), or any federal court, would issue such a ruling, mainly because there are no groups claiming that the Constitution forbids gay marriage/civil unions. So yes, the correct ruling would be, as you say, not to block them, but also not to impose them in any way. The correct ruling -- one we could be fairly confident that Scalia and Thomas would render -- would be that there is no Constitutional right to such arrangements, and so it is therefore for the people and/or states to decide. So on that we seem to agree, but I suspect that O'Connor and Kennedy would not agree, and would istead vote to create a new right, thereby taking the power away from the states.
I may be wrong about O'Connor and Kennedy, but I just can't see them ruling otherwise after Lawrence vs Texas, even though they went out of their way to say that such a decision does not mandate gay marriage. I consider that caveat to be meaningless, and no obstacle to future activism.
As to Congress; well they haven't passed a law banning gay marriage in the country. They have instead tried to protect the states by passing the federal DOMA which says one state does not have to recognize the gay marriages performed in other states. So again I'd agree that Congress should not define marriage for the entire nation. For federal purposes, however, I do think Congress should be the authority on defining marriage, as they did with the other main component of the federal DOMA.
I may have been too hard on O'Connor with regards to the issue of gay marriage. In the Lawrence vs Texas decision she said that the state has a legitimate interest in preserving traditional marriage, but none of the other 5 justices in the majority, including Kennedy, joined her in that concurrence. So that means there is likely a 5-4 majority on the present Court who would impose gay marriage if given the chance.