She's not just a good cocksucker, she's in love. It's her cock and she craves sperm in her mouth. Not all that common, particularly with younger women. Such proclivities are more common among mature slut wives who allow themselves to become carnally addicted to sucking hard cock and shameless sperm gobbling cumsluts. If you find a natural born slut with these appetites, try to hold onto her. Talk to her, compliment her sluttiness. I've found most sluts are proud they've become sluts and if you encourage them they will act even sluttier.