Might have to try a blow job instead

Fucking the alien turned out to be a much more complex process than Ben had imagined. Firstly Big Tits' pussy was incredibly dry and the process of forcing Ben's big cock up there was painful for both of them. When Ben tried the most obvious foreplay he could imagine - playing with its nipples - he found only that his own need grew, the alien simply regarded him with curiosity. Ben surmised that it was the ever-wet alien dog cock that did the lubricating in 'normal' alien sex.

The dog cock was the other problem: As he felt that slick wet thing slide into his asshole, Ben just felt physically sick and his hard on diminished in seconds.

"This is without purpose," sighed the little alien in its sing-song voice. Ben felt a surge of pain as Big Tits activated the collar in a fit of pique. Hell hath no fury like an alien scorned thought Ben, blinking as the pain receded.

"There must be other ways that humans ingest seed," mused the little alien aloud.

"Whoa, hold up," said Ben. "Did you say ingest?"

"Yes," confirmed the alien, "is that not the purpose of sexual intercourse?"

A broad grin spread across Ben's face as he rested his hands on the alien's shoulders and pushed downward. Big Tits felt nervously for the pain controller, but did not activate it yet. Soon it found itself eye to eye with Ben's angry red cock. Big Tits felt Ben's hand pushing on the back of its head, pushing it closer and closer to that huge throbbing cock.

As his cock slid between the alien's lips Ben became pleasantly aware that its mouth was much slicker and wetter than its pussy. At first, Ben was very much the one in charge, pulling on the alien's head and thrusting in and out of its mouth. But Big Tits eventually got the idea and soon proved to be a gifted cocksucker. The little alien would lick and stimulate each part of Ben's cock in turn and scientifically gauge his reaction, soon learning to concentrate on the areas that gave him greatest pleasure.

"Play with your tits," gasped Ben, as the little alien brought him to the edge once more. Big Tits played with its big tits.

Ben watched the alien, as it pinched and tweaked its nipples, bringing no pleasure to itself, but stimulating Ben visually as well as orally. Through its translucent skin, Ben could see the opaque outline of his own cock being massaged and stimulated by the alien mouth.

As Ben came closer and closer to the edge, he closed his eyes. The pleasure from the alien mouth was almost unmanageably intense now and Ben realized that Big Tits had built up such a profile of his manhood now that it could keep him on the edge forever if it chose. In an agony of pleasure, Ben grabbed the alien's head and its mouth, thrusting with all his might. Soon he felt his release as great globs of sperm surged up his shaft and down the alien's throat.

Ingest that, you bastard, he thought grimly.

As he gripped the smooth alien head, Ben felt a strange tickling sensation between his fingers. Something thin and prickly was growing out of the alien scalp. It felt like... hair! In shock Ben opened his eyes. Big Tits' mouth was still impaled on his prick, but now the little alien had a full head of hair. Moreover, its skin was becoming pinker, it seemed taller and its (her!) dog cock was gone.

Curiously, Ben leaned back to examine the face of the alien, if that is what it still was, that was still suckling on his cock.

In doing so, he received his second shock of the day, for he was staring straight into the face of his sister.

"Sue!" he gasped.

The girl who had once been an alien slid her mouth off of Ben's cock.

"Who?" she asked in a sing-song voice. The voice box was Sue's but the accent was alien.

"You're my sister, Sue," Ben breathed incredulously.

"The DNA transfer is complete," commented alien-Sue, "does this not normally happen after sexual intercourse?"

Then Ben realized that it was not quite Sue. There were subtle, but almost indefinable differences (a better attitude towards him for a start). The alien had not so much become his sister, as a female version of himself. Not a confused masculine transgendered version of himself, but Ben as he would have been if he had been born with a couple of X chromosomes, rather than an X & a Y.