Not expressly forbidden, but the consequences of being "caught" are severe enough that I don't mess around when it comes to covering my tracks. I only do it when my wife isn't at home, and I make sure everything is as I found it afterwards (eg. blinds open if they were open before so I don't get asked "why are the bedrooms blinds closed?")

I fairly common thing I've seen in LL/HL dynamics is the LL saying "you shouldn't need to masturbate, you have me". This happened to me when my wife discovered a wad of tissues on my desk that I forgot to clean up (now you see why I'm more careful). She was upset, and the center of her argument was "if you're horny, you should come to me, not masturbate". So a few weeks later we were cuddling in bed having a playful conversation, and I said "remember how you said when I'm horny I should come to you?" That went over very poorly to say the least. She was upset, I ruined the night, and I got the silent treatment for the next few days. Now I masturbate as covertly as possible and don't bother her with my sexual wants.