I have had a great laugh reading through this topic.
I think a reason that I want to bulk up and become muscular is purely for the self esteem side of things having fitness as an added bonus. So ill feel good about my physical appearance, especially when it comes to getting into bed for the first time with my girlfreind. Even though when it does come to that, I realise my physical appearance wont matter for both of us - but what the hey. Hell I wanna look good for the first time she sees me with my shirt off.
When me and my girlfreind break up, picking up chicks later will come as an easier task too, or at least getting their attention.
And all the while ill be as fit as a fiddle! So if I dont get laid at the end of the day, I can still look back and say "Well at least you've got your health".