The Virtue meter determines how strong the heroines' heart shields are in resisting attacks (advances) from the NTR parties. A soft virtue (blue, number 1, 1 to 3 bars) heroine's heart shield breaks on 1 attack, normal virtue (yellow, number 2, 4 to 6 bars) breaks on 2 attacks, hard virtue (red, number 3, 7 to 9 bars) breaks after 3 attacks.
Each heroine has an egg that will only be summoned when they engage with sexual activities. Each egg has a fertilization timer; failure to impregnate the heroine before the timer elapses will reset the egg and the fertilization process. The game is pretty much between how fast can the heroine be impregnated by either you or the NTR parties. The game ends when either egg of a heroine is fertilized - heroine gets pregnant. You get an ending if Mamoru succeeds in impregnating a heroine first; a game over ending if any heroine gets pregnant by the NTR parties.
The orange/pink 愛 is Ai, meaning Love. These pops out when a heroine's Love gauge is full and you get to flirt with the heroine, restoring the heroine's virtue. Orange Ai is a standard flirt trigger; you restore the heroine's virtue and maybe some dialogue lines or repeated sex scenes, nothing more. Pink Ai is an event flirt trigger; a unique event ensues for this flirt process which will progress the heroine's story and the game. Love gauge carries over different times of the day, so you may see an Orange Ai in noon turning to a Pink Ai at night. Blue ゆ is Yu, meaning bath. You will only see this during bath phase where bath action is available. Bath restores heart shields.
To summarize, you try to impregnate a heroine before the NTR parties can. To do this you care for the heroine to gain progress on their Love gauge, Flirt with them (Sex) during Pink Ai and trigger events (Green ?) to progress the story until Mamoru can breed the heroines, at the same time preventing them from being impregnated first by others either through heart shield defenses (bath to restore heart shield, flirt to strengthen virtue), interfering NTR advances by occupying the heroine's activity slot, reset fertilization gauge by having sex with Kyoko at the infirmary during day time or rely on Yukino to guard the heroines.