The sex drive is so strong (usually) in humans that unless you are absolutely physically unable to have sex you will find a way.
I'm fairly thin (6', 175-180 lbs) and have had sex with overweight (50lbs+ over) as well as underweight women. There was no difficulty in either case.
I think the OP may have been referring to people who are morbidly obese. In that case, I have no experience to draw upon; but I'd be willing to bet they get it on just fine.
