Mawaru Penguindrum has always had this elegant, fabulous vibe it. It is sensual, sexy, and, above all, fashionable. Being fashionable allows Mawaru Penguindrum to be super sexual—erotic, even—without seeming sleazy. In anime, this might just be the difference in taste level between, say, shibari in Mawaru Penguindrum and panty shots in some harem anime.
Yes, this feels pretty obvious. Most of us know intuitively what blatant fan service is. But when the sensual scenes are treated with class and taste (à la Hanasaku Iroha), the lines between fan service and non-gratuitous sexual themes blur. For example, how does the bath scene in Hanasaku Iroha differ from the sexy rape in Mawaru Penguindrum? Both are elegant, titillating, fluffy, and aggressively and violently gentle. Is there even really a difference? 
I think so. And, I would further suggest that whether something constitutes fan service is not in the eye of the beholder.
Let us switch gears for a moment. A while ago, I came across this backstage video of Karlie Kloss in a cover shoot for Vogue Italia, December 2011: 
Nudity, sex, and fashion have always been intimately related. Indeed, fashion photo shoots and cover spreads that seem to peddle sex more than clothing are not uncommon—some issues may even feature only women… and nothing else. By comparison, the above video, while provocative and sexy, is actually relatively tame. Yet, there is still a clear line between these fashion shoots and pornography.
At its core, fashion is about selling an image. The clothes, accessories, perfumes, and beauty are all avenues toward that. Furthermore, the audience who desires these products is the same who wants to own that sex appeal, that image. Thus, even in the absence all the luxury, the nudity is still fashionable as long as the image is there. What makes something so erotic still so clearly fashion, then, is simply this: who does the fashionable girl want to be when she takes off the clothes? The motivation behind the shoot and the way the girl presents herself justify its status as fashion.
Likewise, anime is ripe with sexuality. It faces a similar dichotomy in sexual suggestion and fanservice. And, we can apply the same sweeping generalization we used for fashion here.
For example, Mawaru Penguindrum has this super fashionable atmosphere about it. Any sexual themes and nudity depicted build toward this vibe. They sell an image of a confident, dominating Princess of the Crystal as her layers of clothing drop, or a questionably empowered Ringo as she tries to rape her way through fate.
Or, my personal favorite, a vulnerable Yuri bound by her past. These scenes are not gratuitous, regardless of how titillating the audience may find them.  Intent, together with method, elevates the sexuality into something more than mere fan service.
Indeed, as a fashionable anime, Mawaru Penguindrum can get away with showing quite a lot of nudity and skin. In fact, I think Mawaru Penguindrum could be even bolder in a similar vein that, say, Redline is, not just because I find Princess of the Crystal (and everyone) super sexy, but because this is fashion.  I certainly would not mind if she strips off all her clothes as she struts down that runway… In sexy confidence.
- To be fair, the shibari scene involving Ohana from Hanasaku Iroha is very reminiscent of the shibari rape scene involving Ringo and Yuri from Mawaru Penguindrum.
edit: Also, I use classy in terms of aesthetics (subjective to the viewer) not in terms of intent (not subjective to the viewer).
- I have been a huge fan of Karlie Kloss for a while now. This post is partly just an excuse to show this lovely video.
- Oh, is a scene ever so titillating~ ♥
- As an interesting aside, my earlier post on the oral sexual side of Redline had my blog marked as mature.