I'll admit that I don't care for facials, but it's not to do with humiliation or anything like that. I don't want cum in my hair. It's a vain reason, but we all have our quirks, and one of mine is that I don't like my hair getting messed up. Messy from hands running through it is okay, but not messy from cum.
If it was a real turn on for a guy, and he was special to me, then I'd probably let him come on my face, once in awhile.