Big romantic gestures aren't a requirement for deep love. It' perfectly possible to love someone without them and Ianto comes across to me as someone deeply in love with Jack.
Coat sniffing, obvious smarting in Kiss kiss bb, look in Adam.

I don't get that feeling from Jack with regard to Ianto, with regard to Gwen yes, but Ianto no. It's nothing to do with Jack being flirtatious. He could come across as super serious about Ianto to me and I wouldn't expect him to stop flirting. I'm not that bothered about the flippant and I would be even less so if I felt Jack was underneath it all serious. But I don't.