sex question

Okay so my boyfriend is the one who I had sex for the first time with. I'm 18 he's 21. We've been dating for 6 months but anyway. Back a few months ago we had sex and he put his finger in my mouth for me to suck while he was behind me. It turned me on a LOT. After that time I've only ever tried it one time with me initiating it and he said he didn't know if his hands were gross from eating and petting the dog so he didn't want me to. So a few days ago we had crazy awesome sex and I started sucking on his finger and he seemed into it, idk. But after he was like, why do you like sucking my finger? And I said I didn't know I just do. And he asked if I imagined it was another guy...I was like babe no.