Virginity (Trap Quest)
Regardless of what the player thinks about sex upon entering the game, they have no experience with it whatsoever. So, the first time the player experiences sex in a "new way," the game will know, and it will change as a result. Virginities are generally lost in sex scenes involving virginity taking NPC's, although there are some other things that take virginities as well, such as the Lake Monster, Living Belt of Sturdiness, and Vines.
Vaginal Virginity
After spending a long enough time in the game without having vaginal sex, female players will recieve the option to receive bonuses from their virginity. This allows the player to acquire the virgin warrior class, and gives them incrementing stat bonuses that will become stat penalties should they ever lose their cherry. Even if the player does not make the decision to recieve bonuses, only a female virgin can pull the sword in the stone, and the priestess class is far easier to obtain. Should the player lose her virginity, new classes like the succubus become available to her, and the sword of purity transforms into something far less useful.
Anal Virginity
Anal virginity matters more for male players than it does for female players. If the player has never had any "real" sex with a female npc when they lose their anal virginity, they will take a stat penalty, and continue to get them periodically (along with some unfortunate clothes) until they can get rid of their conventional virginity. Until that happens, the player can acquire the Sissy class, which causes npc's to bully the player.
Penis Virginity
The player may lose their penis virginity either by defeating a female monster and having dominant sex with her, or by experiencing submissive penis-play. Note that unsuccessful domination attempts usually don't count, and neither do blowjobs. Anal sex, however, does count.