I think it's clear that sex per se can be, and is, divorced from sexual (or otherwise) relationships... there is no reason to suppose that sex dolls, therefore, are, by definition, bad for relationships... and so be a sin (at least according to Aquinas / Christian Natural Law)... but, equally, that doesn't mean that it is inherently good, either... it's a tool for sex, and could be used as a sex toy in the same way that other sex toys are used... the 'human' shape is misleading, i think...