Reading this thread the thing that keeps popping into my head is how at Tasha's funeral when the holgram version of her is saying all the goodbyes she didnt get to she looks at Data smiles and tells him 'it did happen.'
I think it was, as others have already stated multiple times, a case of regret after seducing probably the only one of her coworkers who wouldnt realise she was under the influence of a mind altering chemical. A fairly new coworker at that since this was only the second episode and the pilot showed that apart from Riker/Troi most of the crew hadnt met before being assigned to the Enterprise together.
Maybe, like Pulaski, she saw Data more as machine than person at first which contributed to her embarassment at the situation. I like to think over the course of the first season as everyone got to know one another better her regret/embarassment lessened especially since data went along with pretending it never happened and didnt treat her any differently. If anything they became good friends.
I always read her holgram telling him that 'it did happen' as an indication that she was no longer regretful or embarassed by what had happened and didnt mind him acknowledging it if he wanted to. By then she knew him well enough to know he would probably never tell others about it unless he had a good reason to.