I'm all for Jack and Ianto being exclusive, as long as it applies to Jack as well as Ianto. There's a tendency in both canon and in fic for Jack to have sexual tension with several different people whilst Ianto only has sexual tension with Jack - and a tiny bit with Captain John. :)

I agree that it's strange that neither Gwen, Tosh, Suzie, Diane etc showed any sexual interest in Ianto and yet Owen seemed to be sex on legs but that's the Torchwood writers for you.