I am bi, so for me this applies to both pussies and assholes. I totally agree that cum is by far the best lube there is. The most enjoyable fucking I have ever done has been in pussies and assholes that already had other men's cum in them. The feeling of that warm slick cum caressing my cock the entire time I am fucking is totally fabulous.
The other subject mentioned in this post was Ass-To-Mouth. I always love to lick and suck a cock that has just pulled out from fucking bareback. I don't really care whether he just pulled out of my asspussy or someone else's. I love the taste of the combination of cum and ass juices or cum and pussy juices.