If your soul is on the line based on what you do on a volleyball court, maybe you should switch to a new religion.
I really feel that this isn't a question that's even asked at higher levels of play. If I were playing in an "A" tournament here in Chicago (real USAV or whatever type "A" - not local park district rec league "A") and held back from serving the female in a co-ed match, she'd probably be both insulted and pissed.
Even at the "A"ish level open gyms that I go to around here, the girls who play that level expect to be treated the same as everyone on the court with a penis. Hell, if they're playing on that court, they're probably better than half the guys there. (We've got a former Women's NCAA national champion and defensive record holder that plays with us occasionally.)
I've never played by any particular "unwritten rules" other than, "play at your level." If I'm a AA/Open level man, I really shouldn't even BE in an "A" level tournament serving at girls. If I'm a "B" level girl, I shouldn't expect preferential treatment if I've signed up for an "A" level tournament. If you're playing above your level, you better fully expect that you could be hammered on every single point.
No matter how much some people would like them to be, tournaments are NOT "make everybody feel good" deals. They're, by nature & definition,
competitive events. You've got no call to expect anyone to go easy on anyone else in the tourney. Unless the rules specifically prohibit the man serving to the girl (a tournament format which I'd never play in, FWIW) it's a perfectly valid move and one which should surprise no one.
That all being said, if some dude was playing below his level and taking advantage of that, he was being a ****.