Consumiendo comes from the verb consumir, which means 'to consume'. While it can mean to use something up, in this context, it carries a more intense and emotional weight.
Here, the singer uses it to express how he is 'wasting away' or being 'consumed' by his unexpressed feelings and longing for a kiss. It vividly portrays the internal suffering and emotional toll of his unspoken love, making it a powerful and memorable word in the song.
"Por Un Beso" is Intocable's confession of a love the singer can't bring himself to speak. He's consumed by wanting one woman — "Que por dentro estoy muriendo por un beso suyo" (that inside I'm dying for one kiss from her) — but the moment she's near he freezes: "Me quedo sin fuerza con ganas de amarle" (I'm left without strength, wanting to love her). He admits she has no idea she fills his thoughts, and that it's fear alone keeping him from bringing her a rose and saying "te quiero" (I love you). At night she takes over his dreams — "Cada noche está en mis sueños" (Every night she's in my dreams) — and he wakes feeling as if she already belongs to him, still stuck wanting to break "las cadenas del maldito miedo" (the chains of this damned fear) for the one kiss he can't work up the nerve to take.