I didn't know either so I went on a quick Google - turns out, conventional latex condoms are soaked in casein to make them smoother [1], and said casein is also responsible for the sour-ish odor.

Vegan condoms and sex toys are made without casein or other animal-based byproducts.

Fun fact: a lot [2] of consumer goods use animal-based byproducts either as technical agents (e.g. beer that's being filtered through fish bladders) or as outright ingredients (shampoo, perfume, ...).

[1] https://www.greencondom.club/vegan

[2] https://www.onegreenplanet.org/animalsandnature/products-you...