“There's a high chance of breaking up if it's my girlfriend”
“But they're just friends”
“I'm sure our friendship will last a long time”
These thoughts serve as an important reminder: what some readers might see as an extreme case of “friends with benefits” (insane manga logic etc.) is actually just working around a trauma caused by an earlier rejection, as seen in the very first chapter.
Basically, if a “girlfriend” eventually wants a break up (or is not even willing to start a relationship at all), then an existing “friendship” is more reliable even if gradually “upgraded” to being sexfriends, and thus this weird harem is their only chance of having a stable and long-lasting relationship.
(There are such harems in the so called real life, like
that one for example.)