[princessmarisa]

Maybe it is a slang or regional variation, but not one I am familiar with.

Usually how you change to the negative depends on the type of adjective used.

I looked it up and sekushi- is a na adjective, セクシー（な）

so plain/casual form would be to put jya nai on the end.

boku wa sekushi- jya nai.
僕はセクシーじゃない。

Would need it clarifying, but to me the sentence doesn't sound quite right without 'yo'　よ on then end either as it is stating quite a powerful/emotional/abrupt statement I think.