"Am I 'cheating?'"

that depends entirely on your partner's definition of "cheating." YMMV.

"If that person's the same gender, is it a homosexual act?"

with regard to sexuality, i think that simple binary oppositions like "heterosexual/homosexual" are becoming obsolete, precisely because of situations like this, and the growing transgender movement. if someone is biologically male, but pretending to be female online, i think it's a similar situation in some ways (but not all) to someone who's biologically female but self-identifies as male, where definitions like "homosexual" or "heterosexual" are simply inadequate to fully communicate the complexities of the situation.

in the case of cybersex, i think such definitions are even less adequate, and i think it's completely absurd to automatically privelege biological sex as the sole determiner of homo vs hetero when you're talking about an online situation.

our current definitions no longer apply. we are in desperate need to a wider vocabulary here.