I think your confusion may be in that you do not realize that the drug does THREE things, not just one.
1. It suppresses ovulation so that, if it hasn't happened, it will be further delayed and a pregnancy may be averted.
2. It blocks fertilization IF it is taken soon enough.
3. It prevents implantation, which is abortive if an ovum was already present and the pill was not taken in time to prevent conception.
I don't think that any who accept contraception as a legitimate would express the least difficulty with the first two effects of Plan B, but the third action is a mortal threat to a newly-formed embreyo. I know a few women who can tell when they are ovulating and MIGHT be able to use Plan B with appropriate discretion and avoid destroying a viable embreyo. By and large, however, this is not the case and this drug does, and will continue to, result in the death of unborn human lives.
THAT is the crux of the issue.
"It prevents implantation, which is abortive if..."
And out of those ways in which it works, that is a big if.