Disclaimer: I Do Not Own Torchwood bebecause if I did Owen, Tosh and Ianto would be very much alive.
He would never stop loving Lisa, but he had space in his heart for Jack; he didn't know if Jack would ever have room in his heart for him.
If, Jack were mortal they both know he would choose Gwen in a heartbeat, but he isn't and he can't bear to watch her wither away like so many others.
Ianto accepts this the same way Jack accepts if there were a way to bring Lisa back so they could be together again he would.
The difference is Ianto is always honest about it, whereas Jack pretends that he doesn't love Gwen.