Nearly all childfree by choice women are OK with abortion. I am pretty sure that CFBC, in the event of a slipup, use the morning after pill; in other cases, they would have an abortion. In any case, the majority of "contraception failures" are "failures to use contraception properly"; if you are motivated and really do use modern birth control methods correctly, the risk of a disaster is not zero, but it is small.
I am personally somewhat skeptical about quite a lot of the endless reams of "accidental pregnancies" we hear about on MN. While accidents can genuinely happen, women who really and truly do not want children nearly always seem to avoid having them.
I think that a lot of pregnancies described as "accidental" fall into the grey area of "not exactly trying, but not preventing, and then dithering and leaving it too late for a termination because I sort of want a kid with this guy and sort of don't."
And I also think quite a lot of them were planned, but the woman is engaging in a little rewriting-of-history when things don't go too well. Whenever we hear on here about a woman struggling with motherhood because her partner is a useless twat (cheat, hopeless with money, gambling/gaming addict, no interest in family life), the pregnancy is always described as "accidental"; I think that in a lot of cases, the real truth is that the woman is now embarrassed about having planned or semi-planned a pregnancy with such an obviously hopeless guy.
"Help, help, I had a contraception disaster" is less mortifying than "Yes, I was a fool and did indeed intentionally get pregnant (or not try very hard to avoid getting pregnant) with a guy who most people could see was not going to be a good father."