Neither one of them is unmarried.
Saraswati is the wife of Brahma the creator god; here is how the Srimad Bhagavatam describes Brahma:
Lord Brahmā, the lord of Sarasvatī, was mystified. “What is this?” he thought, and then he was not even able to see. Lord Kṛṣṇa, understanding Brahmā’s position, then at once removed the curtain of His yoga-māyā.
Brahma also has a second wife Gayatri, by the way.
As far as Kartikeya goes, according to South Indian tradition he has two wives, Valli and Devasena. Valli isn't really mentioned much in Hindu scripture. But regardless of whether you accept South Indian tradition, Devasena is definitely described in Hindu scripture; here is a description of the marriage of Kartikeya and Devasena from the Vana Parva of the Mahabharata:
And considering that this being (Skanda) was undoubtedly destined to be the husband of this lady by Brahma himself, he had her brought there, dressed her with the best apparel. And the vanquisher of Vala then said to Skanda, 'O foremost of gods, this lady was, even before thy birth, destined to be thy bride by that Self-existent Being. Therefore do thou duly accept her lotus-like beautiful right hand with invocation of the (marital) hymns.' Thus told, he duly married her. And Vrihaspati learned in hymns performed the necessary prayers and oblations. She who is called Shashthi, Lakshmi, Asa, Sukhaprada, Sinivali, Kuhu, Saivritti, and Aparajita, is known among men as Devasena, the wife of Skanda. When Skanda became united to Devasena in indissoluble bonds of matrimony, then the gods of prosperity in her own personal embodiment began to serve him with diligence.