I think you just have to know your group.
In some instances it is absolutely not going to go over well, if you have a less mature player for example that wants to side track the entire run every time a pretty NPC flirts with him. Another instance is when players have a real world romance and bring it to the game, nothing wrong with it unless it detracts from the game and we have to sit and listen to your fantasy date instead of playing the game. Absolute worst situation is the GM and his wife/girl friend where they aren't considerate of the other players and carry one with their relationship at the table through NPCs. Especially uncomfortable if they are angry at each other.
But then again I think a good game group with a good GM it adds nothing but an extra dimension to the experience. I have seen basic strangers, but excellent role players, sit down at a table and become best friends or lovers, MOSTLY in game. Really adds a whole new level. NPCs can have the same effect where people are willing to do incredible things for Gwen J'star the extra friendly waitress at their favorite eatery then they wouldn't do for Armorer Loyalty 5/Connections 5.
