It's a terrible idea for a few very simple reasons:
- it would be essentially impossible to prove beyond reasonable doubt (no one monitors pill intake, and such monitoring would be a gross breach of privacy)
- it would result in perfectly innocent women experiencing contraception failure (that would be about 5% every year, in the real world?) being wrongly accused of deception, and very probably pressured into getting an abortion (under threat of being sent to court by their partner)
This second part reminds me of how women experiencing miscarriage get criminalised in certain parts of the world (accused of having had an illegal abortion)
It's also a complete false equivalence. Stealthing introduces the risk of pregnancy and STDs. Both are massive direct health risks, resulting from a substance being introduced into a person's body against their wishes.
Lying about being on the pill introduces a risk of pregnancy, which is of no physical consequences for the "sperm donor" himself. There's legal/financial ramifications for sure, but it is in no way equivalent with injecting an unwanted substance which carries serious health risks into someone else's body.
And most importantly, in term of "monitoring": putting on a condom has no impact on a man's bodily integrity, and can be easily checked on the spot. A condom having been removed is also easy to prove, given the resulting mess. The only "invasive" tests required would involve the presumed victim, not the presumed perpetrator.
The pill having been correctly taken or not is much harder to prove or disprove, and involves the monitoring, over a significant period (including, before the act??), of which substances enteed the presumed perpretrator's body. Monitoring this is neither reasonable nor practical, and could easily result in coercive control from abusive partners, and distorting women's choice about what to do with their own bodies (e.g. opt for the much more invasive implant itself, as it's easier to prove? but it can come with more unpleasant/dangerous side effects)
Bottom line: don't want kids? Wear a condom, get the snip, or fuck off.