Try hibiscrub and very warm water. Also if you put a layer of Vaseline on the inside of the thigh when you are done it won't stick so easily next time, although you will have to redo every couple of days.I think the girl on our yard uses petroleum jelly which is almost the same as Vaseline.
My lad has melanomas up there and he now has regular cleans with the vet - partly because he needs sedating to do it and partly because the melanomas increase the risk of smegma getting caught and infections brewing. It means we can keep an eye on the melanoma size too. Since we've been on this regime, there's been no smell and no gunky legs, plus he seems a lot more comfortable.