How will stress affect sexual desire?

Stress and desire. Dependent on the degree of stress, if you are so preoccupied by life difficulties (sometimes unsurmountable) your time for sex or think of sex will be reduced as a consequence. Stress accumulation triggers a chain of events in the pituitary and thalamus that affects the function of the adrenal glands reducing general health of the person. Sleep problem usually comes with reduction of libido.
Most likely decrease. Psychosocial problems may effect sexual desire and performance, usually negatively. See these references:
It usually decreases. Sexual desire. Stress is a mechanism which is meant to help us deal with tough situations, I.e, fight or flight. Sexual desire takes a backseat when we feel endangered, in addition extended stress leads to fatigue, insomnia and therefore less energy for sex. Ironically, having intimate relations is one means of reducing stress, however, when stressed we may not feel like doing it.