I wouldn't call Sinfest COMPLETE trash....but yeah, it's pretty bad. It's a shame that as soon as the art gets really good, with visually appealing color schemes...the plot totally veers off road.
The author turned feminist along the road (maybe something happened?) and his comic which was mainly about poking fun at various religions and some flirty innuendo, turned into "war on patriarchy".
Monique becoming lesbian was part of that change of focus. It makes sense, because she realizes that all this time, she's been an object and even Slick only sees her as potential sexual partner, not as a person.
Monica turning into a lesbian didn't make sense in Wapsisquare probably because the focus of the story didn't change. The character changed for no good reason.
Obviously, Tatsuya Ishida hates the way women are treated in today's society. From a fun oriented comic, it turned into a very political statement.
The only honest male character is Criminy, a kind bookworm. All of the orthers are slave to their desires.
I can understand where he comes from though, because his criticism of the male-oriented society, catering to their desires in order to make money, often hits home, even though it's not as grim as he makes it to be.