Deffinetly the last one you mentioned, I never understood how someone could feel or see themselves as sexy until I gained, It gives me a level of sexual satisfaction which I don't think i'd ever have been able to obtain being thinner or even being with a girl. A girl who likes it as much as me would just be a tremendous bonus.
Although, I do, on some primal level enjoy the idea of being bigger then a girl i'm with, makes me feel more like a man, which I know is stupid, but its the truth. Also, having an interest in gaining girls, means that the fatter I am, the fatter she can be while I still maintain my caveman need to be the big guy.
Not that I can't appreciate the difference of being with a thinner girl, but i don't think anything could be hotter then being a couple of fat sexy jiggling people together in love.