I was at a catechetical function recently at which a teen asked a question. The question had nothing to do with the lesson and was not some deep theological inquiry. The question itself is unimportant, what is important was that another adult catechist chose to answer the question with the usually unsatisfying "only God knows'" answer.
It's true that there are some mysteries that have not been revealed to us, and so are only known to God. This particular question did not concern one of these cases. As a matter of fact Church teaching on this matter is clearly stated in the CCC. It was not a case of grave matter, so probably no great lasting harm was done. However it was quite obvious that the other person was either unfamiliar with the answer or simply unwilling to tell the teen, perhaps because they saw going down that road as counterproductive to the lesson that was suppose to be covered.
This method of dealing with this kind of issue is generally unsatisfactory for a number of pedagogical and catechetical reasons. First it was poor classroom practice. If the catechist did not know the answer or the Church's stand on the subject then he should have informed the teen that he would get back to her with the answer. If he felt that the discussion would hijack the intended lesson then it would have been far better to tell the teen that he would be glad to discuss it after the meeting. Second it did not fulfill the catechist's responsibility to represent the Church. Though not a matter of grave sin the teen was now misinformed. The Church has a teaching on this subject which the teen and indeed the whole class now believes is open for debate, because if only "God knows for sure" then my opinion on the matter is just as valid as yours.
There is a third option for the catechist's vague answer, and that is perhaps he thought the teen would not like the Church's position. This, unfortunately, was allowed to distort catechetical teaching for many years. Catechist did not talk about the Church's stand on truly grave issues because they feared teens or their parents would react in a negative way to those teachings. This has been true of abortion, remarriage, chastity, and contraception.
I give the catechist the benefit of the doubt, while still pointing out the ultimate problem with this kind of response.