Nana to Kaoru” manga is way too related to this site to not to publish it here. from the description:

Kaoru is a 17-year-old virgin who has an SM fetish. He secretly dreams about an SM relationship with his childhood friend Nana. One day Kaoru’s mom asks Nana to hide all his SM toys so he’ll study for a change. However, Nana finds the leather one-piece that Kaoru bought and tries it on, but she accidentally locks it and doesn’t have the key! The beginning of super strange relationship.

So, let the long fetish and bondage journey begin! See Chapter #1 below.