My petite girlfriend's younger sister... secretly having unprotected sex under her skirt (reverse NTR) He already has a girlfriend but keeps getting penetrated NHDTB-731

My knowledge of sex is limited; I thought my introverted girlfriend suddenly turned into a nymphomaniac! She let people see her underwear, and after kissing, she sat on my lap and penetrated me without a condom! The pleasure was unbearable, and I kept ejaculating inside her!