I don't think it's a problem for a guy to stare you at there since you are the one who wears those kind of 'sexy' shirt.
It's purely normal for a guy attracted to a cleavage just as a woman attracted to Gucci, ot Prada.
I don't think you should feel happy, instead feel proud of having a man staring at you. That shows how elegant you are. We don't treat woman like chicks!