I would say, that yes the slow metabolism argument isnt right but not exactly untrue, since if it was then how can there be plently of slim people who live off take aways and alcohol and get a bus or drive everywhere?
My idea is that external factors take an effect and like I have said before one of the major ones is sex, when I was slim I had a lot of sex with my girlfriend, I also used to be sick on a night out drinking that I did once or twice a week.
Now im single and dont go clubbing more than once a month my weight goes up so easily.
I have been to Mcdonalds twice in 4 months, first time I shared a big mac and chips with a friend, and last time had a mayo chicken.
I often starve during the day but get something full of calories like a few slices of pizza then a yogurt at dinner and drink a lot of fizzy drinks, I do a lot of walking like to uni each day and back, sometimes do an hours walk into town centre then around town for a few hours, dont drink alcohol, dont eat things like crisps or chocolate or sweets etc.
But my weight hardly changes, but on the rare occasion I eat sweets my weight goes up so in that way its metabolic as im not burning off what I am putting on right.