Whether it is her best friend who has caught your eye, her big sister or even your girlfriend's unexpectedly foxy mother, this has to be one of the real hot potatoes in relationship counselling.
As always there are variations on the theme. It could be that you just fancy the idea of the girl in question more than the girl herself. It could be that you are drawn to the notion of some kind of, ahem, threesome. And what could be better than if both girls already know each other and get on?
Possibly you think the two of you are on the way out anyway, and whilst heading for the exit you naturally cast your eye around and happened to spot someone else you liked the look of more. Or maybe you actually are in love with your girlfriend's best friend in which case you really do have a problem and by you I mean all three of you.
Something about nothing?
But if it is not that last thing if it’s that you fancy someone you have no business fancying then the problem is at least containable. Taking a liking to someone other than your partner isn’t that unusual. But as long as you take care not to move it onto the next rung, it really does not have to be that serious.
Most of us have an itch from time to time, after all, and sometimes all it takes to avoid trouble is to keep quiet about the itch and not to scratch it. And frankly the threesome idea is not such a big problem either. You know it is not going to happen, so best file it away under F for 'fantasy' also meaning 'for your eyes only' and leave it at that.
...On the other hand
But the other thing, the falling in love thing, unfortunately that is not something you can just leave lying there in the hope that it goes away. Before you press that big, red, 007 villain-style self-destruct button that is wired directly into your existing relationship, however, you need to make sure you have the full picture.
For example, you might be thinking that her best friend also fancies you, but for her part she might not be thinking anything of the sort. It is possible, for example, that what you see as her flirting with you - and some might not blame you for responding to it - is in reality just her normal warm, outgoing nature.
OK, she might seem to do it a bit more with you than with anyone else, and maybe she should not. But if she does she might be doing it genuinely without realising. Maybe she just does it with you more than with others because as her best friend's boyfriend you are in some way 'safe' as you are already spoken for.
In other words her flirting with you might just be a bit of harmless fun, nothing more significant than that. And if that is the case then no one could really blame her for being genuinely horrified when you decide to take it seriously, or to look upon it as some kind of an invitation to switch horses mid-race?
The reality after all is that you and she are already in some kind of a threesome with your girlfriend, just not the sort you envisaged. Surely having that much in common means she could be forgiven for seeing you as almost one of the gang, rather than an attractive, available testosterone-fuelled member of the opposite sex.
Are you just after something new?
You also need to ask yourself whether you are simply chasing after something new and exciting, because even the most successful relationships have their ups and downs and sometimes we do odd things during the downs.
After a while we also become more attuned to our partners' bodies and moves, which unfortunately can mean slightly habituated to them. Sometimes because of this fortunately not always, and usually only temporarily one can experience a frisson of excitement at the thought of someone different. Not because they are necessarily better, but just because they are new.
But if it is for real?
Then I guess you have to go for it. If you have explored the options, and satisfied yourself that you really do think there is something real going on then maybe you have to make your move.
You will of course make enemies, no matter how carefully you plan it and no matter how well you try to 'handle' it. How many? Your girlfriend, certainly, everyone who cares for her, probably, and quite possibly, eventually, her best friend too. That is because if you and she do not work out in the long-term, and with your record you may not, you can bet you will get the blame from all sides.
It's a high price to pay, but you knew that before you even asked the question.