> A society that permits abortions doesn't prevent an individual from choosing not to have one.

Yes, but the objecting individual is not fighting for his or her own rights. Pro-lifers consider themselves to be fighting for the rights of the unborn child.

Similarly, a society that permits slavery doesn't prevent an individual from choosing not to own slaves.

The society is not forcing its lifestyle on the objecting individual, per se, but it's forcing its lifestyle on the slaves, whom the objecting individual is concerned about.