Boys are (I speak from experience) fickle creatures. The lines that boys to men draw are all different shapes and sizes. This is why, when talking about aesthetic appearances, understand a guy’s evaluation can be tough.
For the sake of conflict, let us say your way of dressing hasn’t changed at all since before you were talking to this guy. If this is true, it is probably also true that he liked your style quite a lot before you began dating. This is because, without labels, guys do not always have the same insecurities about loss, it’s not that you mean less to a guy before you start dating, but what you mean to him can change as emotions get involved.
Guys do have emotions(who knew?). I have many friends who tell me they think their girlfriends are dressing too scantily and it is bothering them. I’m willing to bet ninety percent of the time it is because they are starting to focus too much on the control in the relationship. Guys never want to feel that their girlfriends can walk right out the door and into the arms of another, due to their physical appearance. So, if a girl is drawing attention by what she is(or isn’t) wearing, it can cause a man’s mind to move.
How to help the situation: When getting ready, try asking for your boyfriend’s advice on what you should wear or pick up his opinions on different pieces of clothing, make-up, etc. What this does is help a guy feel comfortable with you trying to be “sexy” because it puts an emphasis on his opinion, thus helping him feel that you are trying to be sexy for him, not just because you want to look sexy. Additionally, it helps you figure out what the specific guy in your life defines as sexy- it is not always the same.
However, you should never feel the need to vastly change yourself or the way you present yourself for a guy just based on his perception of you. A girl who is confident in her image and holds an “I don’t care what people think of my cat sweater, I like it.” attitude is almost always sexy. Unless the cat sweater is really really ugly, but there are surely still guys who like really really ugly cat sweaters.