Each of the 4 main stats (lewd, suffering, degen and service) has related skills to them, but the skill will only unlock after you reached certain number on those stats.
If your service reaches 3k first then your girl will be a slut. If your suffering reaches 3k first then your girl will be masochist. Never managed to find what the degen one does (degeneracy is the hardest stat to level), but lewd one will give you perverted.
If you 'have sex' with a girl until she got one of those skills but doesn't claim her virginity she will get 'debauched virgin' which grants a 20% increase for all stats. Same with Virgin, once you claim her virginity the skill will be erased.
On that note, if you managed to max all of a girl's sex stat you will unlock the final lewd crest: Primal Heart. Easiest girl to reach this is Luce, since she have the pink rotor equipment in her blowjob scene which will raise her clit and vagina skill to 999 if you just leave her alone with the rotor turned to max, meaning you can level her mouth skill while simultaneously leveling her other shill.