The dude bottoming (getting fucked, getting penetrated, taking the cock/dick/penis) above is getting everything done right. He's arched his back in order to maximize his pleasure while being fucked.
In doing this, he also minimizes any discomfort that he may experience. Remember, being fucked is an invasive action. There will always be a little pain, initially, as your sphincter has to adjust to the size of your top's cock (dick, penis). As your top begins to fuck, there is an additional small amount of enlargement of the cock, in addition to the normal erection swelling.
As the bottom partner becomes accustomed to the size of the cock, the minimal amount of discomfort disappears and arching his back, as shown above, allows his prostate to be massaged by his partner's dick, thus offering him immense pleasure. The above posture also allows the top to penetrate deeper which increases the amount of joy both partner's experience.
It you're going through all the effort to get busy with another man, you might as well go for the ultimate pleasure, right?
Take care and stay proud and bare!
ReNude Pride