Harry and Seven are both in most ways such innocents - I think they would be kind of sweet together. Seven is emotionally so young...she really needs more of a boy-girl romance, not a full-fledged adult affair, to help her grow up. And Harry could give that to her. Even in love - heck, especially in love - you need to walk before you can run.
And I think he's cute. He's a cute boy. And yes, Chakotay is drop-dead gorgeous (and Seven is double-drop-dead gorgeous, of course), but that for me wasn't enough to make them "sexy" together. I found them awkward
together. It's not the actors' fault - it's the fault of bad writing and an inadequate build-up.