Yes, can be if Partners have no imagination. Usually, sex becomes boring when one partner has a lover/mistress or pudusm appears and only the goat and missionary position is done. As a colleague says, do a 69 if you're bored. =))
I believe that partners should always find ways to spice up their sexual lives. When you are doing the things over and over again, it is definitely going to feel boring. So, trying out new things in the bedroom is something to be considered.