I've got a very big serve for my level (or likely for most levels), and have played a lot of mixed. I rarely flatten out to the woman, mostly due to not wanting to appear to be any more of a jerk than I already am
. When I do flatten out, I'm aiming at corners and certainly not hitting body serves. I've hit serves that were "flat" for me that crossed the baseline at shoulder height for my opponent, and I've had a 4.5 man duck out of the way of a flat serve (this was after he was previously hit in a sensitive area with a serve earlier in the match). I certainly wouldn't want a 3.5 or 4.0 woman getting pegged, no more than I would any other opponent, but it's a little bit more acceptable if you're playing same-sex than mixed.
It's not just the fact that it seems unsporting, but also, I have yet to meet a woman who has seen enough kick serves that she can judge the bounce and do much, if anything with the return, so that's a higher percentage play anyway.
An effective server should have more tools in his pouch than just the big flat serve anyway - I hit more slice and kick than I do flat playing doubles. The big flat one gets reserved for singles.