Of course he was using her as a means to an end in that setting (being held captive and sent a slave woman to spend the night with). I assume when you say he wasn't "just" using her you mean he enjoyed it as well, thought she was beautiful, had compassion for her etc.. This really doesn't negate the using part. It's kind of repulsive because she's a sex slave, not just a beautiful girl who he sleeps with mutually to further a mission. She's in his room because she was ordered to be there and she cannot say no.
If she would have said no, Kirk wouldn't have forced her, or convinced her to do so at the risk of her own life. Kirk loves women, and he doesn't abuse them. He doesn't use them and cast them aside. In this case, he thought he was going to die, and a lovely woman asked him to be with her.
Was she a sex
slave? Likely. We know she was a slave. Is Kirk the kind of person who would feel right forcing a girl to have sex with him, just to satisfy his wants? I don't think so.
Plus, I just watched that episode the other night, so the details are still fresh. Kirk refused outright at first. Then she cajoled and convinced him, because she likely felt the same way. A good looking, noble man who was likely to die the next day, and so she wanted to bring him comfort. Kirk acquiesced because he did
find her attractive, and he did believe he was likely to die the next day.
Also, to be completely fair, we never saw it, or even the hints of it happen. We saw them kiss, then we saw Kirk fully clothed, laying on a single couch. At least with Elaan of Troyius
, Kirk and the Doleman were seen together, with Kirk putting his boots back on.