Infection!

Monroe felt a mixture of fear, dread, and even anger as he stared down at his huge, throbbing erection. This was not him. This synthetic, metallic body was an impressive mimicry of his, but it was not the real thing. Under different circumstances, he might even enjoy the changes he was experiencing, but not like this. An electrifying jolt passed through his body, disrupting his thoughts as a wave of pleasure made him moan out in bliss. His cock and balls began to swell once more...

The reptile had been augmented by nanites. Metallic and cybernetic components systematically replaced biological organs. The nanites had nearly completed the conversion process; once complete, the host's new body would inevitably pass its progeny on to other organic life forms on its home planet, initiating their conversion in turn. But the changes this male underwent consisted in more than a mere transition from biological to technological. Subtle changes to the mechanical reptile's body conferred certain benefits to the transmission process. Enlarging the host's testicles and increasing their productivity meant a larger fluid deposit with every mating, a greater load to carry the infected nanite cargo. A longer shaft meant deeper deposition into potential new hosts. Additional girth reduced the chance of unintentional leakage from around the thick tool. These augmentations all but guaranteed successful nanite infection for any men Monroe would sleep with.

Other, more drastic changes occurred at the neural level. Maintaining higher cognitive processes would be little more than wasteful expenditure for a progenitor bot. By shutting down unnecessary threads dedicated to superfluous thoughts and emotions, additional processing power could be dedicated to protocols for evaluating potential hosts, swiftly overpowering them, and depositing within them.

If any remnants of his former psyche persist, the lizard will grow in time to accept that these nanites have brought him nothing but pleasure and sexual fulfilment. He will come to accept his new role, as a host and transmitter for the nanites who have taken control of his body.

And if he continues to fight this natural order of things, the nanites can increase the number of threads dedicated to reward processing, flooding him with pleasure until he breaks.