Trust me...you're not. In fact, you are going to have to realize that in the real world, at some point, the BFF will start to fade away as you and the boyfriend get more serious. You'll get to the point where you all hang out on occasion but nothing of frequency.
It's just how things work. It's nobody's fault, its just how things work. Even if your BFF was a chick, you would do the same thing and then you'd both call each other and talk about how you never hang out, only to never hang out or have to make some plan way in advance.
You should however have a talk with the BFF so that there are no hard feelings. While there shouldn't be, what should be and what happens don't always coincide.
Make sure you do stay in communication and what not because when things go awry that's who you're going to call and who you're going to need.
Also, how does the boyfriend feel about the BFF. That could play into as well.
I don't know, my boyfriend has a female best friend, and they still hang out a lot without me. We have been dating for just under three months, so I'm not sure when the bff is supposed to fade away...
Gotta say I totally disagree with the whole BFF fading away over time thing. That's one of the biggest problems I observe in the relationships I witness - the couple becomes one anothers' world and then when they break up their whole world falls apart because they have nothing left.
If this person is your best friend, they need to remain an important aspect of your life. You might not spend AS much time with them, but you should definitely still make time for your best friend. While before you might have hung out several times a week, now maybe its every couple of weeks or once a week - that's ok. But to let that friendship fade away because you are now in a relationship is not healthy.
Make sure your friend understands that your priorities are shifting now that you're in a relationship, but also make sure your bf knows that he can't expect you to give up the other important things and people in your life.
Because you best friend is a guy, this will cause friction in your relationship, at least in the beginning. It's up to you to make sure you bf knows he doesn't need to feel threatened by your best friend and it's important to avoid doing things that would make him feel threatened.
On another note, if you are feeling like you are cheating when you hang out with your male best friend, it's probably a good idea for you to explore those feelings. If your feelings for him truly are platonic, there's no reason you should feel that way....