Sebum, not semen. Its the stuff that makes hair greasy, yet without it hair would be dull and brittle. Its composed of oils, fatty acids, wax esters, squalene and other things.
But what component of sebum actually makes it sticky and clump to things?
Not all oils are sticky. Would it be possible for a creature to have slick sebum instead of oily sebum?
It is not the sebum directly that causes clumping, but the production of excess sebum.