No one has said that 'couple' is like 'police.' 'Police' is always plural; 'couple' is sometimes plural.
However, 'couple' is usually plural with a verb like 'talk,' which is normally an individual action.
>>The couple talks or the couple talk.
>>Either one is correct.
But with the verb 'talk,' the latter is much more common. It's harder to think of situations in which talking would be done by them together as a unit, rather than one at a time individually. Maybe when it's a generic couple rather than a particular couple, and/or both of them talking to a third party: Before the wedding, the couple usually talks to the minister in detail about what will happen in the ceremony. But even that sounds a bit strange to me; I would lean more toward 'talk,' because they don't speak in unison.
>>"Are any people here now? A couple are here already."
This is a different meaning of the word 'couple,' like 'paar' with a small P: Ja, ein paar / zwei schon.
Here's a similar sentence with the meaning we were talking about, 'couple' in the sense of Paar with a capital P:
Are any people here now? Yes, one couple is already here.
Here they're also being considered as a unit, a group of two doing one action (being present). However, you could also say 'are' if you regarded them as two individuals, or if in your mind you were thinking 'Both of them are here.' (Because 'both 'is always plural in English, another surprise to many learners.) In this case, either verb is right, they just express a slightly different perspective.
>>"The wedding couple are going to spend their honeymoon in Las Vegas."
>Sounds plain wrong to me. The wedding couple is going to...
Oh? And just how would you finish that, then? Spend its honeymoon? I don't think so. Here the emphasis is clearly on them as people, not as a social unit. Again, look at MikeE's example. If 'it' does not make sense in the rest of the sentence, you cannot use the singular. If it makes sense to use 'their,' you need to use the plural.
English is just different than German when it comes to singular/plural agreement. Some nouns behave like German, but others do not. Again, please be sure to look in the archive if any of this is surprising to you; we've discussed all this at length before.