We'd been married a year and had nothing but vanilla sex 3 to four times a day. (We were teenagers)
I walked into the kitchen one day and he asked me to turn around, and bend over, and pull my panties down. My face was burning with embarassment, but I complied. To my complete surprise and humiliation he placed his finger covered with butter on my anus and slowly pushed it in. He had never touched me there before. When he saw how aroused it made me, he knew he could demand more, and he did. That was the beginning of the road we have taken. I still get aroused thinking of it 29 years later.