It all boils down to what we, the viewers, really would want to see.  Because Jack himself has endless options.

Remember that he's immortal.  Even if he were to settle into a monogamous relationship he wouldn't really be giving up all that much since any partner would eventually die.  Indeed, he could go on being a serial monogamist for aeons to come and still have a pretty wild and diverse sex life.

Now, that raises the question: is Jack more likely to be monogamous with Gwen than he is with Ianto?  Gwen herself is not an especially faithful person.  Maybe that's a point in her favor in Jack's mind versus Ianto since he figures any relationship with her would be a open one.  Of course, I don't think she thinks that given her possessiveness and the way she clings to Rhys, even when she was cheating with Owen and even now when the question stands of whether she could have Jack.  But Jack may think she's more open than she is.

Ianto on the other hand has demonstrated that his commitment, once gained, is absolute.  The scanty info we have on Ianto suggests that he is likely to be a monogamist.  Now, if I were Jack I wouldn't worry much about that given my own life expectency.  But who knows what he thinks?

However, we're our world here and the question is what is expected of Jack as a character.  Truthfully he actually isn't as much of a sex fiend as his reputation suggests.  I mean, who have we actually seen him have sex with?  Nobody, not even Ianto.  He flirts a lot but we haven't actually seen much action from him besides man-on-man deep kissing with the real Jack and with Ianto.  Not complaining here by the way.  But would the audience be okay with a committed Jack as opposed to an available Jack?  Or do they prefer keeping the fanfic boards humming by hinting at every imaginable possibility?

Also, I find it interesting the way that he drops statements about past homosexual liasons when talking to Gwen.  There was the twin acrobats thing.  Then this last episode there was the "Have you ever eaten alien meat?" thing.  Either he thinks she's likely to be excited by the idea or he's dropping hints to back off.  Not really sure which. 

I wonder how much of that is script and how much is ad lib?  Obviously, Jack is omnisexual but John Barrowman is gay.  Torchwood is probably a relief from his usual leading man roles in that so far most of the kissing has been with men.