Bruh, There's some people reading this here that don't like the way the characters act.
The two couples love each other, there's no doubt about it, but they have desires and that's what Miko is exploring.
Amane really loves Susa, That's why she is agreeing to this and she does have some interest in exploring new things.
Saku likes Susa as a friend as much as he likes Miko as his girlfriend so he's comfortable enough to be in the same bed with him if that's what makes him happy, and well, Saku kinda likes it lol.
Susa still doubts himself because he loves Amane but he still has strong desires towards Miko, which is partly her fault, I mean, she is the one pulling the strings here to get the big D.
If I were to hate on someone It would be Miko, She's doing this only thinking about herself.. But in the other hand this could lead on Susa getting more confident and comfortable with himself, he still sees a big distance in between him and Saku/Miko and by getting closer to them and fulfilling his desires he might realize that that distance doesn't really matter. He has a girlfriend, does what he likes, he might not be as successful as Saku and Miko but in the end they are his friends and are there for him when he needs the most and the same goes for Amane.
And Susa is not the only one with problems, Miko could learn to appreciate more her boyfriend, he does a lot for her and still she focuses too much on sex..
This is not a Sonen manga, but it does have a lot to do with friendship.. and I hope they can be friends till the end.
Oh yeah, thank you so muuch Squig-sama~ ehe >w<