I don't mind at all! In fact I encourage him to, I bought lube especially for him to keep in his office room and make sure he has toys like his Fleshlight and corn starch for dusting. He can look at all the porn he wants, as long as he shares once in a while
Masturbation is healthy, I really do encourage him to do it guilt free any time he feels the need. We've discussed and agreed that sometimes we crave masturbation instead of sex, it's a different experience, or sometimes you just want to get it out of the way and not deal with the process of sex. We're pretty open and fine with it all.