First: I do not like the word sexy so much because its a bit superficial in my opinion. At first glance it refers to someone's appearance only. Of course it is the first thing that we notice about another person before we get to know her/him, but concerning Janeway I am already past the outer appearance for quite a while.
Judging from pictures only, she looks sexy on most of them, though not on all. Some are simply odd (if I was allowed to post one I would happily provide examples).
I think what makes her really irresistable for me is her personality. It is her passion and commitment to be there for her crew, to care for each single one of them, to be strong and never have doubts that she will bring them home, that she will save them. Its her greatest strength and her greatest vulnerability at the same time.
I think this is also the trait that is uniquely Kate Mulgrew's link to Kathryn Janeway. Its truly her passion, her honesty that gives Janeway's character life. There is an openess, an open mindedness about both of them that touches something inside me.
Apart from the personality, I absolutely adore her hands+arms and her face, those high cheekbones and her gracefull neck

. And her hair. But not always. I never know when it is really KM's hair and when it is not.