The suggestion is there is something inconsistent about saying such a thing. After all, we would not say "You should not rob houses, but if you do, don't cause more damage than you need to" or "You should not rape, but if you do, use a condom".
It's true that these are not things we would say. But why not?
There is a subtle distinction we need to make here.
Sometimes it is perfectly reasonable to say: You ought not to X, but if you are going to X, do Y. Other times less reasonable.
Doctor: You ought not to drink, but if you are going to, avoid spirits.
Nothing unreasonable about that. Jolly good advice for certain medical conditions! In fact, if the doctor thought you were likely to drink, it would be irresponsible of him not to add the caveat "but if you are going to....".
If you want a moral example:
Animal rights activist: You ought not to keep pets, but if you are going to, treat them humanely.
That's a reasonable position, and certainly consistent.
This seems odd though:
You ought not to rape, but if you are going to, use a condom.
But why? It is not inconsistent, I think. But it seems a silly thing to say because:
(i) anyone who rapes doesn't give a stuff about their victim, and so won't bother protecting them with a condom. So it's a pointless request (hence rather ridiculous, if safeguarding the victim is the aim). Ditto the remark to potential burglars (if they do have some concern for victims, they'll keep damage to a minimum anyway; if they don't, the advice is pointless). And
(ii) in the animal rights case, the activist recognizes you may not share their fairly unusual moral position, and so it is reasonable for them to add that even if you don't agree with them about keeping pets, you should still treat your pets humanely. But it is odd to add something similar about rape/burglary, as presumably the rapist/burglar knows what he does is wrong.
Note that for the RC Church to say "You ought not to have homosexual sex, but if you are going to, use a condom" would involve (i) no inconsistency, and, indeed, (ii) not even any of oddness of the sort that attaches to the parallel remarks about rape and burglary.
There may be other reasons why the RC feel they should not say such a thing, but to suggest that it's never reasonable to say "Don't do X, but if you are going to, do Y" is just a mistake.
Indeed, "Don't do X, but if you are going to, do Y" is often a reasonable thing to say, and in these above cases where it isn't, what makes it unreasonable does not make it unreasonable for the RC to say "Don't have gay sex, but if you do, use a condom."
POSTSCRIPT 21ST APRIL. A further thought: Note that the RC Church itself issues such qualified moral pronouncements:
You should not sin, but if you are going to sin, make sure you go to confession afterwards!
Note this applies no matter how serious the sin! (WZ: Also note that it is directed specifically at Catholics, just as you suppose the sexual prohibition is).