There is the old standard that is present in each and every relationship where the mood effects your whole level of intimacy. If one partner isn't in the mood for a sexual encounter the other is just out of luck. While this is certainly the choice of one partner most sex experts state that if you put a little effort into it your mood will soon chance. Try it some time and see what happens. The next time your not in the mood for intimacy express that to your partner but ask if they will lay naked with you and talk. Get snuggled down together so that you can feel each others body. Caress each others skin and talk about dreams the two of you have together. Touch each others genitals in a soft and stroking manner as you converse. Then kiss each other softly at first and then build them up to more and more passionate. See what happens, most of the time you might just find your mood shifting and your juices will start flowing. However if that is not the case then the closeness and conversation has been a plus. However I think ninety percent of the time you mood will be affected and you will change your mind on your mood. Take time and enjoy the foreplay and the building it is part of the sexual experience. Think of it this way if you say I am not in the mood actually what you are saying is I'm not putting the effort into it tonight! Is that fair to your partner? And, is it fair to you and your relationship?
Effort, Imagination and Love can work miracles.