I think where people are getting confused is the distinction between sadism and S&M. While sadism is, by definition, taking pleasure at the suffering of others, that is NOT the entire definition of sadism in an S&M relationship. S&M relationships are about mutual respect, and that is clear if you read a manga like The Real Her, where the budding sadist, Yuuka, realizes that she only gets pleasure from giving Moe pain when Moe /wants/ to feel pain (ie. when Moe wants to be yelled at or stepped on), and NOT when she's forcing that pain on Moe when she doesn't want it (ie. when she crushes her mp3 player). On the other side, while Moe is trying to draw out the sadist in Yuuka, she still says all the time that they need to find compromise so they're /both/ happy. Both parties have to be into it in order for it to be a healthy S&M relationship, and I'm glad The Real Her is drawing attention to that.
This manga, however... hoo boy. I think it's safe to call Esumi a sadist, but in the original sense of the word, not the S&M sense. She's constantly bullying and groping and ogling the captain, forcing her to do things and calling her fat, while the captain protests /constantly/. Literally, almost every sentence out of her mouth is trying to refuse her or say no. It's entirely lacking consent. There may be mutual attraction between them, and I'm sure it will grow over time, but just because two people love each other doesn't mean they are magically incapable of being abusive.
If you must call their relationship S&M, you gotta at least admit that it's not healthy. If you enjoy or are turned on by characters like that, that's perfectly fine, since it's just a goofy manga, but please don't try to justify abusive behaviour as "healthy" or "fine", don't tell other people that they shouldn't feel uncomfortable with it, and for the love of Madoka, don't carry that kind of behaviour into your real relationships, unless you are positive that you AND your partner(s) are into it.
Healthy S&M relationships are based on respect, dignity and choice; and Esumi is not letting the captain have /any/ of those.