I still can't tell what's causing her to fall at the beginning. There's a big monster, but blonde red guy kills it fast, so they do post-battle massage, or something?
commander made a deal with him because the monsters kept getting stronger and even blue was struggling, but he's so strong that he defeats them in one hit. So the deal is every time he defeats 14(?) monsters he gets to spend the night with one of the girls.
also its not that fast the 20th time which is the last one in the bathroom with the leopard print bikini and then the pig suit she tells him that kissing is forbidden. but since the 20th time is special she'll allow it "just this time".
Also from the dialogue, 20th time seems to happen after the previous IF story, since pink mentions her promises of coming whenever he calls her even in the middle of a date which what's happening. And the promise to become sexfriends, since red mentions that he didn't defeat any monsters that day, to which she replies that it's ok because they're sexfriends.