I was able to get to the harem route without having to lower the 1st girl's mood. This is how I got harem route:
1. On first playthrough, focus on one girl - I picked the black hair girl first. Get her to 100% affection and pregnant.
2. Pick the 2nd option after game over. This should carry over your affection stats and restart from the beginning. You are now on your 2nd playthrough.
3. Re-confess to the girl you focused on first. Then have sex with her and creampie her.
4. Focus on the other girl, picking the top left choice in your interactions until you can confess. On the turn you can confess, make sure you bring her lust to above 70 - some combination of the middle left teal button (fondle breasts) and the top left red button (headpat) should do it without getting you punched. If you hit 100 you do get punched, so confess as soon as you're above 70.
5. Confess. On the choice, pick the top left choice. If you do it right you shouldn't get punched.