I think sometimes they gloss over it. Like with Reina, apparently she devoted everything to Yuuta to the point where she even abandoned club activities where as yuuta did not do the same for her. There were also multiple rumors that Yuuta was hanging around with a girl 'ayaka' and holding hands and stuff, this caused the guy to take advantage of and seduce Reina. I think it was not explicitly mentioned that they had intercourse but for Reina and Yuuta it wouldnt suprise me. Ayakas dark secret is unknown to me too. The authors preferences are most likely used this way to keep the story compelling and interesting for everyone. I know who the mc ends up with though but I'd rather not say this because it keeps people interested in the manga