My BF actually goes out of his way to give me some "private time".
We agreed when he moved in with me that personal space is still important, and every Sunday, he goes out to run errands and visit friends. We'll joke about giving me time to "do my thing", but it's actually really cool. Sometimes I do, sometimes I don't - but i can if I want to.

He's home during the day by himself, so he's got his own "private time" built in.

It seems that we both understand that masturbation has nothing to do with how we feel about each other or how desirable each other is. It's about the fact that once in a while...it's just different by yourself and sometimes different is good. We have plenty of sex and it's wonderful but each of us does still, once in a while, just want to do our own thing.

It's definitely something I would TALK about. There was a time when I thought that my partner's masturbation meant that I wasn't a good lover or that they didn't love me anymore. I know better now, but I used to feel really insecure! Talk to him. See how he feels about it. See how you feel about it.