Deed Cat, happy and comes out???

Argh i cant believe this - my cat loves to do what we call 'puddings' where he sort of snuggles into things and moves his paws up and down and claws what ever it be. He purrs like crazy while doing this then i role him on to his back to give him a tummy rub and to my surprise his little was sticking out!! I was mortified as he is deed. Why would the feelings be so good that he sticks his out?? Is he feeling ualy relaxed??? Im confuesd. Please help. No stupid answers. Thankyou