"Is it normal to date a girl and be good friends with her best friend? For example, when her best friend spends the night and I find my boxers on her or when she uses my girlfriend's toothbrush and says it should be fine they are best friends. What do you think?"
Hey man, it worked for Jack Tripper, although I think he was secretly gay. Not that there's anything wrong with that...I digress. It sounds like you are semi-okay with this so I'm not sure where the problem lies. As long as you don't mind that her bestie is up in your grill wearing your clothes and potentially using YOUR toothbrush, then share the love homey. However, if you are not okay with the way things are headed then you should man up and say something to your lady. If you don't, you will surely end up hating her best friend. Which really is no good in the long run because if you start hating the best friend the girlfriend will be in the middle. You can't ask a chick to pick between her boy toy and her bestie, it just ain't right. But, you can ask the bestie to respect your boundaries. You and your girl need to get on the same page and set up some limitations.
Soooooo, I have to assume that as of right now your girlfriend is okay with this also? Her bestie always coming around wearing your undies and such? Because this makes me think that maybe your girly might be setting you up for some three way lovin'. No really, just think about it. Maybe your girlfriend plays for both teams bro and one of these nights they are both going to get you liquored up and take complete advantage of your skinny, white body. I know this doesn't sound half bad but trust me when I say that this little scenario doesn't usually play out the way you want it to. Before you know it, your girl and her bestie are leaving your pasty ass and taking half your stuff with them. Beware the menage a trois, my friend, BEWARE.......
And that is...WHAT I THINK! HA!
