It irritated me badly, the way they did that, and I might be biased, but I thought Ianto didn't display anywhere near the sort of animosity that Gwen did. He was nervous, but that was possibly because he just didn't know where he stood with Jack, something that was addressed when Jack asked him out on a date.
I also believe Ianto was never naive enough to think Jack could or would restrict himself to loving just one person. Estelle was a prime example, there.

That, and Ianto was just too content and comfortable around Jack in the next episode for nothing positive to have happened between them.

Thank you. :)