I've always held that there is a massive difference between two people who are in a loving relationship together, and two people who are simply stuffing body parts into one another.
You have your relationship and your love, and that isn't going away - but can enjoy the random, meaningless sex too.... Great! I'd guess that after fucking a FB for so long, you're starting to feel a little off-put by it's regularity. Perhaps it almost feels like you're blurring the FB/relationship distinction if it goes on too long.
I don't think that this is weird at all. You're just keeping the random, meaningless fucks... well, random and meaningless!!