My fantasy would be... A guy invites me to his place, and while there, he has me slip out of my panties and sit on the sofa. His dog comes in from another room and begins sniffing me, and the guy encourages me to spread my legs, assuring me that things are fine. Then the dog starts licking my pussy all over, getting me aroused until I'm nearly begging for cock. Then I get down onto the floor, bending myself over the sofa, while the dog mounts me and thrusts inside me. After the dog finishes, the guy tells me how beautiful I looked, as he fingers and caresses my clit and doggy cum-filled pussy.
Otherwise, I want to be fucked in public, so that I can be dripping cum all day. Not so much "free use" or anything like that, I'd either want to be fucked periodically by the same partner while we're out and about, or keep meeting people over the course of the day who turn me on and don't have the patience to find a hotel room or something.