Theres levels to it. A girl who is solid, average, or chubby is very normal and while they have some fat they can be as great in bed as a more athletic woman. The truely obese is another matter, they cross the line and stop being sexy (to most guys, there are guys with fat fetishes). Obese men and women have a lot of restrictions on sex, a lot of positions won't work, you can't be in different locations and due to poor health they will often tire quickly. In the case of guys the bigger you become the higher the chances of impotence as the fat in the blood stream hampers circulation.