I didn't watch it as Ianto the sextoy...I read it in the 1st season as Ianto distracting Jack from figuring out about Lisa. Sort of protective/self-sacrifice but the being attracted to Jack started before he was even hired (as we saw in Fragments). In the 2nd series, they've actually grown attached to each other as seen in Adam; they've been affectionate even during work hours.

Plus Ianto's a badass...they just needed to work more with his character. I mean he probably tasered a guy to death.