A slang word usually used in yaoi (male/male intimate relationships) culture, made by combining the Japanese terms "seme" (the dominant one) and "uke" (the submissive one). It describes a person that enjoys both positions in a relationship and moves between the two based on his mood, the mood of his partner, and the situation itself.
OMG, thanks for pointing that out. *o* How could i have missed it??
I've just re-read this and realized the guys from the first couplé were sekes !!! ヾ(☆▽☆)
http://www.mangago.me/read-manga/bond_z/an/bond-z--chapter-1.html/33/