Stress can put a major damper on your sex life, both lowering your libido and getting in the way of orgasming—not to mention the strain it can put on a relationship in general.
Research has even shown that experiencing stress is negatively correlated with sexual activity (both in terms of sexual behavior and satisfaction). In one 2010 study on sex and stress, the researchers write, "Higher self-reported stress in daily life was associated with lower levels of sexual activity and satisfaction and a decrease in relationship satisfaction."
It's also not uncommon for a stressful thought mid-coitus to totally take you out of the moment (which can be hard to get back into once you've lost the vibe).
"Stress makes the body produce cortisol, and cortisol interrupts all the happy hormones of sexual arousal," mindfulness coach Emma Michelle Dixon, Ph.D., previously wrote for mbg. "When we're stressed during sex, we also tend to get too much in our heads instead of focusing on our bodies, which is key to experiencing pleasure."