Of course, sex is not just "for only thin attractive people", because that would rule out many people who are having sex, along with many people who are not thin but still very attractive, depending on one's interest. However, your self-image and negative feelings are at play here and affect your life in general. To help that, without doing anything extreme, it would be good to at least walk for 30 minutes per day, preferably 60 minutes, if you can find the time.