When I had a girlfriend, we did it all the time, she said she enjoyed it most when she was on her period.
I realised it was true, when she was on her period our encounters were more pleasurable, she was very creative and her orgasms would come one after the other, I ended up liking doing it more when she was on her period.
Everyone thinks differently, but if you haven’t tried it, I recommend trying it at least once, you won’t regret it.
At the moment I don’t have a girlfriend, but if I did, I would do that.