1. the act or process of falling into an inferior condition or state; deterioration; decay: Some historians hold that the fall of Rome can be attributed to internal decadence.
2. moral degeneration or decay; turpitude.
3. unrestrained or excessive self-indulgence.
4. ( often initial capital letter ) the decadent movement in literature.
Putting aside the merits of the claim for a moment, I think the argument that abortion represents decadence is an entirely coherent and cognizable one.
If you think of a fetus as at least the moral equivalent of human child with some rights of a person,* then definition #3 pretty much covers why someone might regard it as decedent for the most prosperous society in the history of the world to sacrifice the lives of (the moral equivalents of) children in order to facilitate a marginally increased satisfaction of the material desires of some (many) adults.
Its a claim that could be questioned empirically -- why do abortions actually happen? are they merely serving self-indulgent interests? Its a claim that could be questioned normatively -- the very word decadence implies that excessive indulgence is morally inferior, but is this true? and what counts as excessive? You could even ask whether the decadence inheres in the fact of abortion or in the facts that create the incentives for abortion.
But it seems to me that none of these objections stop the initial claim from being rhetorically valid.
* = and seriously, can we just have a rule
where we expect all intelligent people to 1) recognize that when "pro-lifers" say the word "child" they frequently mean the term inclusive of fetuses, and that when "pro-choicers" say the word "child" they frequently mean the term exclusive of fetuses; 2) take the meaning plainly conveyed by any particular use of the word "child" without having to agree (or be seen as agreeing) that the word is being correctly used; and 3) try to avoid getting into pointless semantic arguments by declining to completely alter the implicit assumptions of the other party when addressing secondary points that clearly depend on those assumptions?