Spoilers for up to Day Three, no further
I found the whole propositioning conversation in Day Three delightfully similar to the stopwatch conversation in They Keep Killing Suzie.
... Because, really, the proposition doesn't just start at the "plenty of things you can do with a stopwatch" or "might as well make the most of it, then", but the whole context of it and surrounding conversation.
This might be fanwanking it too far, but I'm really loving the dynamic they're building with this relationship. Mainly I love that it doesn't toss aside the creepy and fucked-upness of Torchwood (and subsequently, both their personalities/characters) to play out the more media stereotypical behaviours of 'romance'.
Like, Ianto's whole "So, you were blown to pieces. Did it hurt? No really, did it?" (matched with the whole joking-about-the-gruesome-death-of-a-coworker-over-her-corpse in TKKS) is his way of showing Jack, very honestly, that he's totally not bugged at all by the horror of Jack's (and Torchwood) life.
But at the same time, Jack *is* a bit bugged by it-- at the start of the "you 'sploded!" conversation, he's clearly a bit uncomfortable. But Ianto isn't relentless unwittingly. I totally love the fact that he's completely, brazenly honest about who and what Jack is-- he's not so much trying to "make Jack a better person" through trying to superimpose some present moral code on him, but constantly accepting and facing the reality of Jack's life--which is pretty perpetually horrific--without flinching, head on.
Because he knows that this kind of thing is *going* to be relentless for Jack, so rather than give him empty platitudes of comfort, he just helps Jack accept it by accepting it himself. And with positive reinforcement! i.e. "You died horribly, and will continue to die horribly forever, even after I'm dead. SO LET'S HAVE LOTS 30 MINUTES OF SEX RIGHT NOW!" -- See also, yeah, TKKS in which Jack is pretty awful/monstrous and Ianto propositions him over a corpse (that probably represents his future demise, too).
Which, frankly, I think is all very consistent with Ianto's characterisation as well--thinking back to his refusal to give up on Lisa even when she had turned into something monstrous, and beyond that, was running around killing people.
But I wouldn't say that Ianto is just swayed to the moral ground of whoever he presently loves. I think he just has his own shady moral code that he very fiercely sticks to--he's loyal to Jack, but disobeys him frequently when he doesn't agree with Jack's orders.
Anyway, ultimately I think that these bits of Ianto's character are what ultimately make he and Jack the OTP. I think Jack is absolutely complimented and kept interested by Ianto's complete acceptance of the shady, horrific reality of Jack's personality and existence. In addition to the fact that that acceptance is through Ianto being completely his own man, ultimately, and it being absolutely his independent choice to be like that and be with Jack, rather than any thrall Jack might have over him. They're both very much their own people.
And that's also why they're both switches. And also why delicateflower!Ianto characterisation fic really, really doesn't work for me.