Since thou wast great in zeal for godly religion, thou didst assemble an alliance of Martyrs, and in their midst, thou shonest like a flashing star. With the arrows that did pierce thy much-suffering body, thou didst slay the enemy, O Great Martyr Sebastian; and thou thyself didst fly as from a bow into the Heavens, where Christ hath received thy soul.
Now, as much as I may not like the idea of a Holy Martyr being co-opted for a decidedly carnal purpose, I must admit that the tendency of Western religious art has helped to encourage this. The sober, sacred, and hieratic charactre that Western religious art originally had in common with the Byzantine tradition (even, for a time, after the Schism) had been replaced by a purely sensual, dramatic emphasis. Whereas the oldest depiction of St Sebastian is a very sober, spiritual mosaic at Ravenna, Italy, this one is all too typical of later portrayals.
As an addendum, I just discovered when reading the infallible Adalbert de Vogüé today (in Reading St Benedict: Reflections on the Rule, trans. Colette Friedlander [Kalamazoo, MI: Cistercian, 1994], p. 82) that in RB 7:24, St Benedict, following the Rule of the Master 7:45, quotes the following maxim from the Acts of St Sebastian 14 (printed in Migne, PL 17):
quia mors secus introitum delectationis posita est;
for death lies close by the gate of delight;
(The Rule of St Benedict in Latin and English, trans. Abbot Justin McCann [Ft Collins, CO: Roman Catholic Books, n.d.], pp. 40-1)