Rough description of the mechanics. The girls and guys will appear at points on the map. If a girl is by herself you will interact with her alone usually. If she with a guy then you can either watch or interrupt their interaction. Your choices there will cause different increases in the stats. From what I gathered you have to at last interacted with a girl once positively in order to take advantage of the excitement stat. If shes excited shes more likely to H. Goal is to get the pregnancy stat to 100, the bottom circle.
Theres a guide back on like the first few pages which has basic walk through if ya need it.
Thanks for taking the time to give a rough description.
A few things wrong though:
1 - If a girl is by herself early game, it just means you haven't had any interaction with the guy and/or other girl present in the scene. Late game this changes if you completely won over the girl, the guy will not appear. However before that point there is always a guy with her.
2 - The part "From what you gathered" is incorrect. Excitement is increased when something sexual happens to her, in order to take advantage she has to have at least 15 affection for you. This means you can take advantage of Toko and Airi from the get go. An is the one exception, requiring 25 aff or waiting til day 9.
3 - To have sex you need following stats: 15 affection (first stat), 60 excitement (second) and 20 development (third). Anything lower and she will just punish you instead when you ask. Again exception with An who requires 30 affection or waiting til day 9 where her requirements become 15-60-20.
4 - This one isn't really wrong but more like incomplete. One of the goals is indeed to impregnate the girls but simply having sex with them, fully winning them over and keeping them away for the other guys are the other three. The game gives you something of an evaluation at the end that changes depending on what you've done with the girls. Complete with bad results say if you impregnate a girl without winning her over.