vi.1. Prajapati, being desirous of propagation, underwent penance; from him when heated were born five, Agni, Vayu, Aditya, Candramas, and Usas as fifth. He said to them, 'Do ye also practise fervour.' They consecrated themselves; then when they had consecrated themsleves and had acquired fervour, Usas, offspring of Prajapati, taking the form of an Apsaras, came out in front of them; to her their minds inclined; they poured out seed; they went to Prajapati, their father, and said, 'We have poured out seed; let it not remain here.' Prajapati made a golden bowl, an arrow breadth in height and similar in breadth; in it he poured the seed; then arose he of a thousand eyes, of a thousand feet, with a thousand fitted (arrows).Kausitaki Brahmana, Adhyaya VI.1, describing the birth of Rudra
So, basically, Usas is freakin' hot, because either she was so damn beautiful that her brothers either started masturbating, or they just ejaculated on the spot.
I'm inclined to go with the latter. You know, from context. Totally. Yeah, that's it. Context.