I went on a hot spring trip with my mother and aunt (who is slender and has big breasts)... "It's okay because we're just kids," my aunt said, so I decided to go into the women's bath with her. Just when I was feeling safe alone with my aunt, a female guest (a female student) came in and I was surrounded by a bunch of breasts. Being a breast lover, I couldn't resist and got a full erection! My aunt noticed my erection and secretly gave me a handjob in the bathtub! But my erection didn't go away... so my aunt had sex with me in a blind spot in the open-air bath and made me ejaculate! My excitement didn't subside until I was left alone with my aunt...