Person A uses the phrase "eager beaver" to mean an enthusiastic person. Person B chuckles. Basically my question boils down to who the weirdo is, so to speak:
A, because "eager beaver" is outdated and the sexual connotations are too strong to hope that one should understand the "normal" meaning
B, because "eager beaver" is a perfectly benign phrase with no sexual meanings whatsoever. Anyone who sees sexual connotations in this phrase is simply being immature.