What dose it means when a guy put his finger in a girl's mouth and why dose he like it so much?

So my boyfriend is a really nice and respectful kinda shy guy. Those days We were cuddling at the beach and he was playng and touching my lips with his fingers. At one point i gentile bite one of his fingers just to play. He stoped for a second and slowly push his finger deeper in my mouth. I was kinda amused by that and I just bite it again and suck it a little. Then he kinda moaned and take it out and told me to stop. I asked If he dont like it but he told me he likes it and it turns him on but there are people arrownd and is not apropriate because of the meanig of that gesture. I asked what does it means but he didn't want to tell me... So what it's the dial with that?
What dose it means when a guy put his finger in a girl's mouth and why dose he like it so much?
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