Not really a common issue in art, but something that could be explored in your fiction maybe. If your alien clown girlfriend is really fat as has trouble moving around, just take her into space and fuck her in zero g. Or maybe just lower g. Unfortunately physics still exist in space, so you're not gonna be able to fling her around like a doll just because the gravity is lower. It DOES make it a lot easier for her to bounce on you though. Maybe this is only useful if you're fucking someone as big as Jabba The Hutt. Or something even bigger...