I read before that some people are hyper-sexual and some people are hypo-sexual. That is just the way they are wired or their nature I guess. Maybe when the novelty of the relationship starts wear down your sex drive will start to wane a bit too. That is usually the case with many couples. If it is becoming a problem with your boy friend maybe you can figure out other enjoyable things to do together such as tennis, ping pong, Frisbee etc. Good physical exercise that is fun and you can do together might act as some what of a substitute and also help you fall asleep at night. Taking a break between bouts in bed will make those special times more special also.
Take care and enjoy life ~ Shoe