does my female dog thinks she’s male?

i have a jack russell terrier thats a half a year old, and a shepherd mastiff mix thats around two to the years old. both are female, and the shepherd mastiff is fixed. i don't what what this is supposed to be, but i caught the terrier humping the mastiff. im very confused. is my terrier doing this because she's not fixed? she thinks she's male? i'll take her being gay as an answer if thats even possible. it sounds silly because i never heard of a gay dog, but i'll take it if thats the answer