In our 40s and 50s, testosterone and oestrogen levels both decrease, which may make it more difficult to get aroused. A decrease in oestrogen can also lead to vaginal dryness. Lower levels of oestrogen lead to a drop in blood supply in the vagina, which can then negatively affect vaginal lubrication. It can also lead to thinning of the vaginal wall, known as vaginal atrophy. Vaginal dryness and atrophy often lead to discomfort during sex. 

Other physical changes during menopause might also affect your libido ~ Hot flushes and Night Sweats, Mood and Irritability, Sleep.