I'll preface this by saying that I realize most people don't care about this, it might just be me because of my personal things, but I have seen other people agree with me in a few threads and other games (not a lot though, I admit).
It's definetely not that bad for most people, it's just that the idea that Bliss is going after the Mom and is romantically interested in other girls kinda puts the MC in a complete sideline, he's just "another piece of meat", specially considering it seems she made a move on the Mom first (which makes it seem she's more interested in girls and more interested in the Mom).
And if Petal or Rose and the MC are so in love with each other, the idea of Bliss coming and seducing Petal or Rose to make her less focused on the MC disturbs me.
Some games do this really well, making the threesomes just a stimulant for the relationship and a nice experiment, but the moment the girls start loving each other it makes me fell like the MC is just an extra. How could Petal say all those things about the MC but then love someone else? Would she say the same things to them? The thought makes me distressed.
I realize this makes me sound COMPLETELY off the rails or something, I'm sorry for trying to have this. It's just that these games are a fantasy and it seems the fantasy of having girls only in love with you is really rare, almost all Harem games tend to be Open Harems (or at least the really good ones, like yours, with good renders and good writing).
Edit: A good thought exercise to have - imagine if Bliss was a guy. Wouldn't everyone in the comments be throwing a tantrum crying "NTR! NTR!"?