Sissies are basically Weak if we go by the meaning, it is being weaker tendencies mostly in men who want to be dominated by women.
Sissies can be crossdressers, slaves, maids, whores etc these are the categories under the sissy umbrella one mostly takes interest in.
A sissy is the sub in the relation with a dom treating them badly like they can make a sissy thier personal whore and make money, they can make a sissy a maid and get thier housework done without paying, can make them a crossdresser and acquire all thier wealth and make them live a life of a beggar or a looser under thier feet.
Sissies generally have fetishes like leather fetish, humiliation fetish, feet fetish, heel fetish, boot fetish, cock fetish.
Why would someone live a life like that, which has no self respect ? Tbh it's just like heterosexual people like being with other gender in a relation and it gives them a turn on, similarly sissies have the thing for all these things and it's thier ultimate turnon for thier sex drive. And it is not compulsary for a sissy to be gay, they can be heterosexual, although thier master/mistress can do pegging and make them suck dicks