>>844Samefag as
>>843 , I initially thought of the "falling in love with the insect monster", but I really like your symbiosis idea! It could also involve transformation. And for that I see 2 case scenarios :
1) A falls for B, the Brundlefly, but the more time they spend together, the more A has bodily side effects (because contagion).
2) A & B are originally humans, at least one does science and stuff. Shit goes wrong in the lab, except it's not noticeable right away (ex: spores floating in the air) but it affects their biology. They progressively both transform and fall in love as they share the struggle.
You could also have A & B not like each other initially, for more tension, regardless of setting. In the end, they both need each other. Not sure about the "fusing bodies" though, it's a more spiritual relationship then, is it?