Negation of Homosexuality is a positive thing, then. Okay.
Exactly. You don't say "No Homo" unless you're worried someone might think you're gay. Insecurities are inherently negative.
Words mean things. Telling people that the nice thing you're doing is absolutely NOT to be construed as anything homosexual means not being percieved as gay is a priority, a CONCERN. Why would that possibly be a concern if you weren't insecure/worried by the mere concept of being mistaken for a gay person?
Words mean things but it is the connotations which provide the actual message of the term, "no homo."
At any rate it's a pretty safe bet there are insecurities at play. But that wouldn't be inherent to the term.