Date a Kaiju!

"The world has been invaded by a massive beast, more ancient and powerful then anything mankind has ever faced. Capable of destroying entire cities and even nations by itself and immune to even humanities most powerful weapons, it seemed as if all hope was lost. At least until researchers discovered something, the beasts DNA wasn't to far detached from that of humans, in fact it was close enough to give them an idea. Despite it's monstrous appearance, the creature was close enough to mankind for cross breeding to be possible, in theory at least. The researchers believed if they could acquire a sample of some reproductive material, they may be able to breed a new kind of super soldier capable of battling the creature! With their plan set, they only needed a way to attract the beast, some way to arouse it's interest in something beyond destruction, and thus the crudely named THOT Corp was formed, gathering the most beautiful, brainy, and talented women the worlds militaries had to offer with the singular goal of seducing a 400 foot tall monster."

I think the idea speaks for itself, MC would be playing the monster, YC would be one or more of the girls in the unit tasked with seducing him. He'll seem like a mindless brute at first before eventually revealing high levels of intelligence as well as other abilities. To solve the incredible size difference we'll either have him shrink or the girl(s) grow, more details can be discussed via messages. I also have a related idea involving the hybrid offspring instead, if giant monsters don't float your boat.