Stereogenic and chiral centers are not necessarily the same thing.
"A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.[1" - Wiki.
"A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer" - Wiki.
Sp2 carbons cannot be chiral centers, since inverting any two groups will not give you an enantiomer. [Exception you probably don't need to care about: allenes]
However they CAN be stereogenic centers. If you have a cis (Z) alkene, interchanging two groups would give you a trans (E) alkene, which will be a stereoisomer (a diastereomer, in fact).
So there ya go. - J