This might not be what you want to hear, but eating junk food is not going to help you become a sexy hunk that pulls the babes. You need to start doing exercise and the type of exercise that builds muscle if you want to gain weight.
Women generally don't like men who are too muscular, so you don't have to look like a bodybuilder. Martial arts is the best sport to do for an all-round good-for-sex body. When I was practicing martial arts, my girlfriend could not get enough of me. I think she made us orgasm about 3 to 6 times per day over weekends (we had to make a living too) and she readily admitted that it was my lean and mean fighting body that drove her wild. I wasn't complaining
The best way to start training is to look for a dojo near you and start from there...