I don't think that's entirely true, I mean hardened Leliana is okay with a bit of fun on the side here and there with the Warden's leave but we saw in Leliana's Song that she wasn't particularly fond of Marjolaine jumping into bed with anyone else, bard or no. She's pretty possessive.
Well, that's why I used 'could be.' Leliana isn't too keen on the idea with the Warden if she's unhardened in DA:O. Whether or not she's polyamorous in that game depends on how her personality is shaped.
As for Leliana's Song...depending on how you view it Leliana getting jealous about Marjolaine either doesn't jive with what the player has been told previously, or it does but is an example of Leliana being slightly hypocritical. After all Leliana (hardened or unhardened) can tell the Warden in DA:O that she's also slept with people as a means of spycraft, or part of an assassination plot, or whatever else it is that Bards get up to. Unless there was a period of time during her career as a Bard that Leliana was not involved with Marjolaine, it would seem that Leliana, at least in that instance.. was holding Marjolaine to slightly different standards than herself. I've always got the impression though that Marjolaine was Leliana's lover the entire team she was a bard, and that's more or less how she got pulled into that life.