If a secular society criminalizes discriminating against a ceremony (e.g., a wedding between arayot [forbidden sexual unions, e.g., incest, bestiality, or homosexuality] or an idolatrous ritual), is a Jewish business allowed to provide a service, such as acting as an officiating Rabbi for the service, baking a cake (custom or neutral), catering the ceremony or designing it? If not, would a kashrut agency be allowed to certify a caterer that, under duress, did? Do we distinguish between Jew and Gentile regarding this halacha?
There are two concerns here: chanufa, which means telling a sinner that you approve of their sinful action; and mesayea / lifnei iver, being involved in (or enabling) someone else's sin.
For a rabbi to officiate at a wedding prohibited by halacha would be an issue of chanufa, as he's declaring okay that which the Torah says is not.
For the caterer, florist, or the like, we go back to the Gemara (Avoda Zara 13b) and Shulchan Aruch (Yoreh Deah 151:1). We don't approve of pagans burning frankincense to their idols. But you are allowed to be a frankincense wholesaler, even if you know that a lot of that frankincense will in turn be sold for pagan worship. Similarly the Shulchan Aruch says that as long as you're not the only frankincense store in town, you're not "enabling" as they could have bought from someone else, so you can (if necessary, though not ideal) sell retail directly to someone saying "I want to buy frankincense to sacrifice it to my idol." (And none of the commentaries there seem concerned that your sale of the frankincense implies official endorsement of his idolatry. You're just the shopkeeper.)
The subject of kosher certification where other parts of an event are objectionable (e.g. the food is fine, but the entertainment is problematic) has been addressed at length by Israeli poskim, with R' Ovadiah Yosef zt'l in favor of having a certification saying "at least the food is kosher."
So the kosher certification can simply say "we're certifying the food, not commenting on anything else"; and the florist and caterer simply put a notice on their paperwork: "Shmerel's Catering [or better yet, Rabbi Michael Broyde suggests you call it "Stanley's Catering"] offers no theological validation", and you've done your part.
The same goes for the wedding hall. Rav Moshe Feinstein was asked (Igros Moshe YD1:72) about a wedding hall in London hosting a wedding between a Jewish man and Jewish woman, in which there would be mixed dancing, or the wedding would be during the Nine Days. He allows it in cases of significant need. Nowhere is he concerned with questions of approval/validation/chanufa; the only concern is involvement. At the heart of the heter is the reality that in a big city, if you turn them down, they will find some other hall, so you're not truly enabling. Rav Moshe suggests several other heterim in his situation that wouldn't necessarily apply here (e.g. there's a net gain in mitzva observance because everyone's eating kosher who otherwise wouldn't be), but the key heter remains.
Note that "I'm just facilitating the sin, not enabling it, as there are other providers in town" is a valid dispensation on the books, but a simple reading of Shulchan Aruch would indicate it's not super-preferable unless necessary. (E.g. the Talmud (Nedarim 62b) totally allows selling firewood to a pagan temple, but employs the argument of "eh it's probably to heat the building, not for a pagan sacrifice" rather than fall back on the "someone else in town sells firewood" excuse.) Traditionally the case of "necessity" would be a significant loss of business, but it stands to reason that another case of necessity -- allowing the other-providers-in-town fallback -- would be "I'd get more than enough business selling incense exclusively to monotheists, but I would get sued if I discriminated against pagans."
If society compelled a rabbi to officiate a wedding prohibited by halacha, could he circumvent the prohibition of chanufa by proclaiming "while I personally believe this is prohibited by halacha, the state compels me to declare you married"? That would be a really tricky question.
To summarize the responses so far, in addition to the 2 potential issues (cited by @Shalom in his answer) of "...lo thitein michshol" (which might not apply if the sin would be committed regardless by other means), and chanufa (tolerance/approbation of sin), there is also a rabbinic prohibition of mesaye'ah (aiding and abetting a sinner which [as noted by @ gt6989b] is an issue even if there are alternative means available). However, the biggest issue (as noted by @wfb) is likely the problem of sha'ath hashmad, when a non-Torah government enacts laws with the purpose of negating Torah values, where the halacha actually demands martyrdom rather than catering the authorities (see, e.g., Maimonides' Laws of the Fundamentals of Torah 5:4).