I guess it depends on how you define 'obese'. If it's clinically, well, that wouldn't prevent a woman from variety during sex. Even if the woman is bigger, that really doesn't have a lot to do with sexual ability. Personally, I've always been a very flexible woman and even now with 50lbs to lose, I am still very flexible. The skinny, fit, or average-sized chicks have always been amazed. I had an ex-girl who was 5'4" and about 100lbs who could not get fucked bent at the waist with her hands on the floor though I could. I can still do that, as well as, put my legs behind my neck, and so on. With that said, unless we are talking about obese as in 'reality TV shows', I am not of the mindset that weight is a big hindurance.
Funny thing is...I run into a lot of guys seeking to have sex with bigger women. A lot say the sex with smaller chicks is lacking and there have been various reasons why ranging from someone women not building their skill because they are smaller and assumably more attractive, thereby, feeling just the fact they have maintained that and are sharing the ass, a guy should be grateful to guys not being able to girls just being too thin and fragile for somethings. They say bigger women are more exploring and confident sexually plus many are more skilled because they've had to work harder in appealing to partners and maintaining relationships (this is just what I've been told).
With that said, I'm what they consider here in Colorado as a small BBW. I am 5'6", 214lbs and shaped like a pear. I wear a 14-16 top and a 18-20 bottom. I am working daily on losing weight just so that I can build my endurance for other things I am pursuing (dance, volleyball, rollerblading) not to catch up with this society's view of 'beauty'. I don't want to be a small chick. I've always been hippy with more ass than average and I want to continue to be that. I just want to get my ass in a size 16 bottom and I'm content.