Spoiler: They do get to have a happy ending but the last volumen is the most sickening and disturbing of all them all. Yuuji becomes fully a woman he grows breasts and his body totally transforms. She/He gives births to twins when Yui it's like 6 years old. WARNING! If you don't like rape but have been able to get through the manga until Volume 3 (Genshi Sairai), I strongly recommend you to think twice about reading the last volume (10). Yuuji gets raped in the worst way you can even imagine.
Animal X