Hmm. If you're in a pretend relationship and you have a tongue match with another girl, is that cheating or pretend cheating...? Call me a purist of sorts, but the wishy washy-ness of the MC is starting to irk me. If it doesn't go the good communicative poly route, I might just drop and come back to see what the ending is.
In a harem manga, it's either that, or the MC has to be suuuper dumb (to the point of not being able to understand when she gets confessed), otherwise the whole setup collapses, and she just ends up dating with whoever was the first to confess.
To keep it as a harem, all five girls need to have something that stops them from becoming a couple with Mei. I can accept that Mei, wants to accept her confession from the heart and not just in the heat of the moment (although she does consider just going with the flow). Of course that's a time-limited excuse, but I don't think the manga wants to drag it out too much, once the other girls confess, it's going to change to "Ok, now who should I go out with?"