I have to admit, and please don't be offended by that, but I had to chuckle when I read your post. Why? Heck, half of my female friends have masochistic tendencies. A bunch of my female friends complain openly about their partner not being willing to give them a good spanking or something similar. One of my closer friends is about to quit her relationship for that reason alone: that she can't gain any sexual satisfaction with her partner who refuses to be anything but nice to her. She is so frustrated and sad about this. And no, they're not freaks or anything. They're wonderful caring intelligent and beautiful people. My current partner has masochistic tendencies as well. I'm not sadistic myself, but I think it's nice to know that a spanking is one of those many things that please her, and why shouldn't I give her that every now and then if she enjoys it?
A monster? Hah.

What does that have to do with being a monster? Sexual preferences are like preferences for certain food. Am I a monster that I like raspberries more than strawberries? You'd be a monster if you'd inflict pain on women who wouldn't want it. But to a partner who wants it, there ain't the slightest problem with it. Two consenting people can do about anything to each other. Who cares? As long as they're happy.
Of course all this depends on your age and on the society and the social class you live in and grew up in. I grew up in a very open minded home, and I live now in one of the most open minded society in Germany. And as open as I in such matters are also my friends. If I'd tell any of them that I'd have sadistic tendencies, which I don't, they'd cheer at me for that.
Seriously: as long as you don't do anything to someone they don't want, the only problem with such preferences are the ones in your head, originating in your upbringing and experiences.
I think it's also important to make sure that we talk about sadism in matters of sexuality. Not sadism in form of someone enjoying a small child tripping or whatever.