The thing is it's Starfleet
that is all hinky about the sex. Lots of workplace rules leads to an atmosphere where people feeling free about their sexuality is seen as potentially boundary pushing. There's a cascade effect to that workplace morality, if you can't do it with your closest friends you sure can't do it with aliens without being looked at askance. So in order for Shelby to not have any preoccupations about how it looks to have liaisons with aliens she has to be bucking the system she's steeped in.