Were the Romans thinking it was only gay if you were on the receiving end a sign of cultural strength or decline?

Of course, even though they're one of the more famous cultures to have "declined", that's only if you artificially don't consider Byzantine Rome, and there's plenty of people living in Rome to this day.

Bimodal is not binary. Obviously there are two ‘modes’ - ie peaks in the probability distribution function, male and female, but there’s a lot of overlap in between. The term ‘multimodal’ doesn’t mean ‘bimodal with overlap’ lol, it means that there are multiple distinct peaks in the probability distribution function.

Look up a bimodal distribution for clarification.