Whether it's gay or heterosexual sex, back pain is normal. Tension builds up in the pelvic floor, the muscles are directly connected to the pelvic floor muscle network. If penetration occurs without sufficient relaxation, lubrication, or is performed too quickly, these muscles contract strongly as a reflex. That pelvic tension radiates directly to the lower back. The receiving partner often assumes positions where they arch their lower back sharply or keep their legs raised for a long time, which strains the lower back muscles. The mental focus required to relax that area causes unconscious stiffness in the rest of the body. And besides, manga artists include it because it adds to the comedy after the “first time.” o.o
I’m glad you’ve had good, pain-free experiences—there’s a lot to learn from that :D I’m actually pretty sensitive to pain; when my partner holds me tightly, I even end up with bruises, but that’s because I’m too soft I guess ;-; From what my straight friends have told me, they said their pelvis and back hurt during long-time sex, but they pretended to enjoy it so their boyfriends wouldn’t notice. Well—I’ve heard that a lot. I also read that the ovaries and uterus are closely connected to the spine, so keeping that in mind... Anyway, big respect to those who have anal sex—because outside of fiction, I can’t imagine doing that without dying lol

Why do almost all manga mention back pain after sex? Seriously, I've never seen that before. Where did this plot device come from? I can only imagine myself having back pain after sex when I'm around 60 years old, and I'm quite sedentary and inflexible.