No, they cannot. The use of sexual devices (“sex toys” or “marital aids”), even in marriage, is gravely immoral. There are a number of reasons why this type of act is intrinsically evil and morally illicit.
First, this type of sexual act is non-unitive and non-procreative. And the Magisterium teaches that each and every sexual act in a marriage must be unitive and procreative. Thus, any sexual act which is inherently non-unitive or inherently non-procreative is intrinsically evil.
Second, a sexual act does not cease to be a sexual act, if climax is lacking. Such devices cannot be used as mere “stimulation” prior to the natural marital act. As explained in yesterday’s post (Q. 1), the normal performance of the marital sexual act itself does not justify “all the rest, in whatever way it is done”, not even acts done “in the preparation” (foreplay). And St. Alphonsus Liguori teaches that sodomy is still sodomy if climax is lacking (Moral Theology, On Matrimony, Book VI, Q. 916).
Third, the use of a sex toy is a type of masturbation, even when the person using the device is one’s spouse. And masturbation has been condemned by the ordinary and universal Magisterium as intrinsically evil and always gravely immoral.
Fourth, the way that these sexual devices are generally used, very unfortunately by Catholic spouses, is to bring the wife to climax outside of the natural marital act. But the Magisterium has condemned any and all types of sexual acts which bring the wife (or the husband) to climax outside of natural marital relations. This includes acts which occur just before or just after the natural act.
Pope Pius XII: “By the force of this law of nature, the human person does not possess the right and power to the full exercise of the sexual faculty, directly intended, except when he performs the conjugal act according to the norms defined and imposed by nature itself. Outside of this natural act, it is not even given within the matrimonial right itself to enjoy this sexual faculty fully.” 
The right to make use of the sexual faculty, within marriage, is not “assigned to the will of human persons”. This means that the spouses cannot perform any type of sexual act to which the two spouses agree. Rather, there are limits imposed by God and natural law. The sexual acts of married persons are moral only when each spouse “performs the conjugal act according to the norms defined and imposed by nature itself.” Any sexual acts which are contrary to “the norms defined and imposed by nature” are unnatural sexual acts and are therefore intrinsically evil. These types of acts, which by definition are inherently non-procreative, do not become moral when used in marriage — no matter what the purpose might be.
Pope Pius XII: “What has been said up to this point concerning the intrinsic evil of any full use of the generative power outside the natural conjugal act applies in the same way when the acts are of married persons or of unmarried persons, whether the full exercise of the genital organs is done by the man or the woman, or by both parties acting together; whether it is done by manual touches or by the interruption of the conjugal act; for this is always an act contrary to nature and intrinsically evil.” 
The Pontiff states that neither the husband nor the wife may attain sexual climax outside of the natural marital act: neither by manual touches, nor by the interruption of the sexual act. This teaching clearly refutes the popular claim that the wife may reach climax by manual touches (or by oral stimulation, or sex toys, etc.), if she does not reach climax during the natural act. Such a claim is absolutely rejected by the Roman Catholic Magisterium. For this type of act is unnatural and intrinsically evil.
Notice, too, that the Pontiff rejects the popular excuse, that these unnatural sexual acts occur about the same time as the natural act. For he says that even if this unnatural act attains sexual climax immediately after the natural marital act is interrupted, it is still contrary to nature and gravely immoral. And this implies the condemnation of another popular claim, that all the sexual acts of the marital bedroom are morally one act. If that were so, then Pope Pius XII would not have rejected unnatural sexual acts occurring immediately after the natural marital act is interrupted.
 Pope Pius XII, Address to the Second World Congress on Fertility and Sterility, n. 22.
 Pope Pius XII, Address to the Second World Congress on Fertility and Sterility, n. 25.
I must also point out that the Holy See has condemned the use of sodomy as gravely immoral for the husband and for the wife. There is no separate set of rules for the wife in the marital bedroom. (Pope Pius XII makes that clear in n. 25 above.) And the Church, in condemning sodomy, thereby condemns any and all unnatural sexual acts. For sodomy has a broader meaning that people sometimes assume.
“If, however, the husband wishes to commit the crime of the Sodomites with her, since sodomitic intercourse is against nature on the part of both spouses who are united in this way and, in the judgment of all the learned teachers, is gravely evil, there is clearly no motive, not even to avoid death, that would permit the wife legitimately to carry out such a shameless act with her husband.” [Denzinger 3634]
The term “sodomy” has a broad meaning, which includes any unnatural sexual acts. And “no motive” permits the wife to agree to “such a shameless act”, not even the motive to prepare for natural marital relations. This teaching contradicts those who say that the purpose (or motive) to prepare for the natural act justifies marital sodomy. It also contradicts those who say that oral, manual, or artificial means may be used for that same motive.
Consider the sin of Onan in the Bible. Sometimes, it is claimed that his sin was only that of contraception. But, in some cases, the onanism can refer, more broadly, to a range of unnatural sexual acts between a man and a woman. In his article The Sin of Onan Revisited, Fr. Brian W. Harrison explains:
” ‘Onanism’, the term derived from Genesis 38:9-10 which in traditional Christian usage has designated both masturbation and unnatural intercourse between a man and woman, is not exactly a pleasant theme to write about.”
“The classical Jewish commentators — who can scarcely be accused of ignorance regarding Hebrew language, customs, law, and biblical literary genres — certainly saw in this passage of Scripture a condemnation of both unnatural intercourse and masturbation as such.”
Onan’s sin is sometimes used to refer to contraception, or to unnatural sexual acts between man and woman, or to masturbation, since all these acts are “intrinsically sterile forms of genital activity” (as Fr. Harrison notes in his article).
The conclusion that we must draw from all of the above teachings is that any type of sexual act, other than natural marital relations open to life, is intrinsically evil and therefore not permissible for any motive or in any circumstance.
More on marital chastity in my book: The Catholic Marriage Bed
Please take a look at this list of my books and booklets, and see if any topic interests you.