I don't think he really had a grasp of the concept of commitment at that point. We hear him say "I love you" to all of his girlfriends in the show--even up through Hollywood Follies. I think that's really important. He has, in his mind's eye somewhere, an idealized version of his parents' semi-normal turn-of-the-century relationship. I don't think he understands completely what it means--it's just a combination of him thinking it's something he should say, hoping it'll please the girl in question, and hoping it'll get him closer to what he wants.
I was gonna make a point here about how Indy messed things up with Kate in
Scandal of 1920 (she'd've been the best pick by far out of the three women in that movie) by interpreting their relationship in Mata Hari's terms...but the next thing you know I'd be writing syrupy fan-fic and drawing a bubble bath.
*takes a moment to remember what Indiana Jones is all about*
Ok. I think I'm much better now.
TC