I'm not so sure this is something you should be concerned with in the beginning of your dance instruction. The sensual, sexy attitude you refer to comes from a confidence witin when you know you have skill. While you could certainly take some styling classes for women, I think when you're first starting out, your concerns should be more about the basics and fundamentals of each dance. I'm guessing since you're just starting out, you're not planning any shows or exhibitions any time soon, right?
It's the gentlemen's responsibility to dance WITH the lady, and not for her. He shouldn't make you uncomfortable.
Another thing to keep in mind... typically, dancers have a leaning towards either standard/smooth dances, or Latin/Rythm. I think it's a personality thing. It might be in your nature to like waltz, foxtrot, tango, etc. more than cha, rumba, bolero. So, don't let it bother you too much that you don't have the same confidence as your instructor. I guarantee he didn't develop that ability after a few months of lessons, and neither will you or any one else.
Hope this helps