A friend writes and asks me why I prefer Jack/Ianto over Jack/Gwen. I've really thought about it because the idea of  "shipping" -- well, let's just say I found fandom by doing a random search in Google looking for information about the series. Slash --what's that? But from the beginning, I was fascinated by the Jack/Ianto dynamic.

Partly it has to do with the two actors playing beautifully off each other. Let's face it, John Barrowman ain't never going to win an Oscar (although God knows weirder things have happened on Oscar night!), and Gareth David-Lloyd shows promise but hasn't really earned his chops yet. BUT... when they are together, they are believable. They are open to each other, and they trust each other (the convention pics prove it, I think), and so make us believe in the characters' vulnerability to each other. They have created two beautiful, damaged people coping the best they can, and maybe falling for each other in the process. It just happened that it's two men, rather than a man and a woman. I don't get anywhere near a similar vibe from John and Eve. They work well together but the vibe between them has a certain degree of distance. The way they play Jack and Gwen, to paraphrase another fictional alien, "the having would not be nearly as satisfying as the wanting."

Partly it has to do with my "read" of Jack Harkness. When you first meet Jack, he's a conman and an intergalactic slut, and he makes no bones about it. But in his own way he has limits. You get the feeling that he would never have tried to come between the Doctor and Rose -- he loves and respects them both. Although he would be happy to jump into bed with either one, he wouldn't do it in a way that would hurt the other. In fact, his ideal would be jumping into bed with both of them. Jack may carve notches on his bedpost, but he does it honestly. And when Jack looks at Gwen, he also sees Rhys. And he sees Ianto, and he loves Ianto, and he knows how Ianto would feel. It's not that he's not tempted -- he's JACK -- but he sees the damage he can do to people he cares about. So he doesn't.

But mostly it has to do with my feelings about infidelity. NO. Sorry, NO. Hell, NO. I have never been able to understand those doomed passionate illicit affairs. If you love someone so madly that you would violate your marriage vows for him -- walk away. Honestly. If you look around and decide that, even though you love that someone, you would rather stay in your current relationship, then stay. Honestly. Don't try to have it both ways. I had issues about Gwen's affair with Owen, but I could understand it; and you get the feeling that it was a quick madness and it passed and she faced herself and chose. If she were to betray Rhys (her husband) and Ianto (a good friend and colleague) by carrying on with Jack, she would become something ugly. 
 
Of course, since this is all fiction, your mileage may vary. Jack can be interpreted in a thousand different ways. You may feel differently about romantic love. You may believe in Madame Bovary. Me, she made me tired.
edited several times to try and make sense of it -- which means it probably doesn't.