Apparently it was explained (about Ronon) in a scene that was filmed and ended up on the cutting room floor. I hope it's alright to post it here, it will (hopefully, fingers crossed) be in the deleted scenes on the DVDs, and it's not spoilery as it's not canon.

Carter: So...you seeing anyone?
Keller: What?
Carter: Around the base, you seeing anyone?
Keller: I dunno...I had a moment with - with this...guy. He’s not exactly easy to read so...I guess the short answer is “no”. You?
Carter: Well, I’m the boss, so I can’t really...
Keller: Right Anyone back home?
Carter: Uhm...
Keller: Un-huh, I thought so. Give it up.
Carter: Well, it’s complicated.
Keller: Show me a relationship that isn’t.
Carter: He’s in Washington...I’m here.
Keller: Ouch. Long distance relationship.
Carter: He’s going to retire soon, so maybe -
Keller: Really! Retire? So...an older man, huh?
Carter: Not that much older.
Keller: Washington, older man...is he like a Senator or something? Someone famous? Would I know him?
Carter: Probably not.

As you can imagine, the S/J shippers are going crazy over this!