Wow, it's really odd to see how quickly opinions change here. So far it also seemed Masaa is a good person to me, but considering this manga is all about superstitious appearance and the truth behind it I#d advise to be careful with any judgements. We see a lot of not nice behavior (cheating, gluttony & porn), but Amano uses this to then turn you around by showing you what is behind it. Which makes it a really good work imo.
Btw, calling Asuna a nymphomaniac might be a little drastic (especially if you also want to imply promiscuity, i.e. "maximizing the number of partners"). Perhaps she has a higher sex drive than some (definitely higher than Fue), but whether it's manic or not is hard to tell so far. Take a look at http://dynasty-scans.com/chapters/the_feelings_we_all_must_endure_ch02#16 for example. Asuna tries, very politely, careful to instigate something and Fue immediately brushes her off. Asuna directly retreats. I think she does love Fue, but she's inexperienced with a real relationship (keep in mind all the girls more or less just entered adult life). She tries to do all for her beloved and not to bother her at all to the point of neglecting her own needs. Just like modern romance tales teach you to. Finally she kind of can't take it anymore and cheats. That doesn't make the act of cheating any better and it doesn't mean she "cheats because she loves Fue so much" (which does indeed sound like a bad porn script). No, she ultimately cheats because she strained herself too much. Stupid tactic. She was too afraid of confronting her girlfriend with her own needs. Sure, there might have been no solution at all (if Fue can't compromise, i.e. if they can't find a level of sex they are both comfortable with), but then they could have split up with a less messy cut.
I think this manga is all about that stuff. Things are not as black and white as they seem. People doing ugly things can actually be pitiful and easy to sympathize with, nobody is just an asshole out of pure spite.
Ruki is on the way of doing the same mistake as Asuna, by the way. Regardless of whether Sacchan is even able to fall in love with her or not (if she's just totally straight Ruki is out of luck I guess), in this example Ruki should perhaps have asked Sacchan to put the bigger problem first, i.e. tell her about the whole issue. But no, she didn't want to "bother Sacchan" and tried to only do what she wants (Ruki liosten to her) up until the point she literally zoned out due to the strain on her own mind.
I understand why she did it, does that make me sympathize with her at all? no because the fact is she still did it and i stated before that i hate cheaters.