Could also be the expression one has while having sexual relations with another... noun.
b) John and Kate are MTB, they always have their sex faces on when they are around each other.
Example 2: Seinfeld: "I don't think I can date her anymore, George."
George: "Jerry, why not? She's perfect for you!! You said so yourself!"
Seinfeld: "Yes, I know, George. I did say that but.....How do I say this...it's...it's her sexface George, it's like she checks out of her body and someone else moves in, and that new person's face is not like hers and it is not very attractive (Jerry contorts his face in an effort to show George) AND she doesn't seem to care or notice. I noticed it in the beginning, I thought I could get used to it, or it might change once the newness wore off, but it didnt, and I've decided I just can't do it anymore. Her sexface scares me, George."
Example 3: "Every time I see her sexface, her mouth forms the shape of a perfect hexagon. It's really impressive."