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.
If the guy wants to make a political statement, well, that's his choice. The problem is Sinfest's format and foundations don't offer a solid platform for those statements.
Writing is all over the place and in what I want to think is Tats' eagerness to represent feminism, it only ends with shallow plots suffering a lot of "telling not showing", horrible pacing, narrative bending and being retconned by the statements instead of the statement being organically included within the narrative.
He tries to make the sisterhood relatable and the new heroes of the strip but they never face any obstacle and in fact, plenty of plots have been caused by them.
