SoS Core does not ship with a full skeleton. It requires . The Core tells the game to look for specific bones in the skeleton—specifically NPC Genitals and NPC GenitalsScrotum . If those bones are missing (i.e., you forgot XPMSSE), the male body will stretch into terrifying, spine-shattering polygons (the infamous "giraffe neck" or "origami crotch" bug).
To ensure stability and avoid the "T-Pose" or "No Schlong" glitches often discussed on Nexus Mods Forums : schlongs of skyrim core