Boyfriend's best friend.....

So my bf and I have been dating for a year and a half and we both couldn't be happier together. We just got an apartment about 3 months ago and are just having things going really well. However, I am kind of having an issue with his best friend since they were in high school.

The thing is I was friends with my bf's friends before he and I started talking. This one guy (name is Vinny) and I were always really flirtacious and just goofy. We knew we were never going to do anything about it because he had a gf and whatnot. Plus I don't have any type of feelings for the guy other than a brotherly figure. Even after my bf and I got together he still wants to be flirty which my boyfriend doesn't mind because he knows I'm coming home with him. But his friend and I lately aeem to have this sexual attraction... :/ I don't know how to put it. I sooo soo so so in love with my boyfriend and I don't ever want to hurt him but I dont know why I feel like this! I don't know if it's because sex between us has been slow cause he's always tired...maybe it's because his friend is always going out of his way to say cute things when sometimes my bf doesn't. Does it make me a bad person that I sometimes think about having sex with him? I always told myself I would never cheat because I've been cheated on too many times....I feel like such an asshole for thinking it...what should I do? :/

P.s. I am 19 and my bf is 30 so there's a big sexual drive age gap....Sometimes I wonder if I am thinking about it since sex with us is so slow....I don't know! Does this make me a horrible gf??